…n test. (#4407)

### Problem

Currently, on Linux the first thin client call to the daemon can deadlock just after the pantsd->fork->pantsd-runner workflow. Connecting to the process with `gdb` reveals a deadlock in the following stack in the `post_fork` `drop` of the `CpuPool`:

```
#0  0x00007f63f04c31bd in __lll_lock_wait () from /lib64/libpthread.so.0
No symbol table info available.
#1  0x00007f63f04c0ded in pthread_cond_signal@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
No symbol table info available.
#2  0x00007f63d3cfa438 in notify_one ()
    at /buildslave/rust-buildbot/slave/stable-dist-rustc-linux/build/src/libstd/sys/unix/condvar.rs:52
No locals.
#3  notify_one () at /buildslave/rust-buildbot/slave/stable-dist-rustc-linux/build/src/libstd/sys_common/condvar.rs:39
No locals.
#4  notify_one () at /buildslave/rust-buildbot/slave/stable-dist-rustc-linux/build/src/libstd/sync/condvar.rs:208
No locals.
#5  std::thread::{{impl}}::unpark () at /buildslave/rust-buildbot/slave/stable-dist-rustc-linux/build/src/libstd/thread/mod.rs:633
No locals.
#6  0x00007f63d3c583d1 in crossbeam::sync::ms_queue::{{impl}}::push<futures_cpupool::Message> (self=<optimized out>, t=...)
    at /home/kwilson/.cache/pants/rust-toolchain/registry/src/github.com-1ecc6299db9ec823/crossbeam-0.2.10/src/sync/ms_queue.rs:178
        guard = <optimized out>
        self = <optimized out>
#7  0x00007f63d3c588ed in futures_cpupool::{{impl}}::drop (self=<optimized out>)
    at /home/kwilson/.cache/pants/rust-toolchain/git/checkouts/futures-rs-a4f11d094efefb0a/f7e6bc8/futures-cpupool/src/lib.rs:236
        self = 0x37547a0
#8  0x00007f63d3be871c in engine::fs::{{impl}}::post_fork (self=0x3754778) at /home/kwilson/dev/pants/src/rust/engine/src/fs.rs:355
        self = 0x3754778
#9  0x00007f63d3be10e4 in engine::context::{{impl}}::post_fork (self=0x37545b0)
    at /home/kwilson/dev/pants/src/rust/engine/src/context.rs:93
        self = 0x37545b0
#10 0x00007f63d3c0de5a in {{closure}} (scheduler=<optimized out>) at /home/kwilson/dev/pants/src/rust/engine/src/lib.rs:275
        scheduler = 0x3740580
#11 with_scheduler<closure,()> (scheduler_ptr=<optimized out>, f=...) at /home/kwilson/dev/pants/src/rust/engine/src/lib.rs:584
        scheduler = 0x3740580
        scheduler_ptr = 0x3740580
#12 engine::scheduler_post_fork (scheduler_ptr=0x3740580) at /home/kwilson/dev/pants/src/rust/engine/src/lib.rs:274
        scheduler_ptr = 0x3740580
#13 0x00007f63d3c1be8c in _cffi_f_scheduler_post_fork (self=<optimized out>, arg0=0x35798f0) at src/cffi/native_engine.c:2234
        _save = 0x34a65a0
        x0 = 0x3740580
        datasize = <optimized out>
#14 0x00007f63f07b5a62 in PyEval_EvalFrameEx () from /lib64/libpython2.7.so.1.0
```

This presents as a hang in the thin client, because the pailgun socket is left open in the pantsd-runner.

### Solution

Add pre-fork hooks and tear down the `CpuPool` instances prior to forking and rebuilding them.

### Result

Can no longer reproduce the hang.

…n test. (pantsbuild#4407)

### Problem

Currently, on Linux the first thin client call to the daemon can deadlock just after the pantsd->fork->pantsd-runner workflow. Connecting to the process with `gdb` reveals a deadlock in the following stack in the `post_fork` `drop` of the `CpuPool`:

```
#0  0x00007f63f04c31bd in __lll_lock_wait () from /lib64/libpthread.so.0
No symbol table info available.
pantsbuild#1  0x00007f63f04c0ded in pthread_cond_signal@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
No symbol table info available.
pantsbuild#2  0x00007f63d3cfa438 in notify_one ()
    at /buildslave/rust-buildbot/slave/stable-dist-rustc-linux/build/src/libstd/sys/unix/condvar.rs:52
No locals.
pantsbuild#3  notify_one () at /buildslave/rust-buildbot/slave/stable-dist-rustc-linux/build/src/libstd/sys_common/condvar.rs:39
No locals.
pantsbuild#4  notify_one () at /buildslave/rust-buildbot/slave/stable-dist-rustc-linux/build/src/libstd/sync/condvar.rs:208
No locals.
pantsbuild#5  std::thread::{{impl}}::unpark () at /buildslave/rust-buildbot/slave/stable-dist-rustc-linux/build/src/libstd/thread/mod.rs:633
No locals.
pantsbuild#6  0x00007f63d3c583d1 in crossbeam::sync::ms_queue::{{impl}}::push<futures_cpupool::Message> (self=<optimized out>, t=...)
    at /home/kwilson/.cache/pants/rust-toolchain/registry/src/github.com-1ecc6299db9ec823/crossbeam-0.2.10/src/sync/ms_queue.rs:178
        guard = <optimized out>
        self = <optimized out>
pantsbuild#7  0x00007f63d3c588ed in futures_cpupool::{{impl}}::drop (self=<optimized out>)
    at /home/kwilson/.cache/pants/rust-toolchain/git/checkouts/futures-rs-a4f11d094efefb0a/f7e6bc8/futures-cpupool/src/lib.rs:236
        self = 0x37547a0
pantsbuild#8  0x00007f63d3be871c in engine::fs::{{impl}}::post_fork (self=0x3754778) at /home/kwilson/dev/pants/src/rust/engine/src/fs.rs:355
        self = 0x3754778
pantsbuild#9  0x00007f63d3be10e4 in engine::context::{{impl}}::post_fork (self=0x37545b0)
    at /home/kwilson/dev/pants/src/rust/engine/src/context.rs:93
        self = 0x37545b0
pantsbuild#10 0x00007f63d3c0de5a in {{closure}} (scheduler=<optimized out>) at /home/kwilson/dev/pants/src/rust/engine/src/lib.rs:275
        scheduler = 0x3740580
pantsbuild#11 with_scheduler<closure,()> (scheduler_ptr=<optimized out>, f=...) at /home/kwilson/dev/pants/src/rust/engine/src/lib.rs:584
        scheduler = 0x3740580
        scheduler_ptr = 0x3740580
pantsbuild#12 engine::scheduler_post_fork (scheduler_ptr=0x3740580) at /home/kwilson/dev/pants/src/rust/engine/src/lib.rs:274
        scheduler_ptr = 0x3740580
pantsbuild#13 0x00007f63d3c1be8c in _cffi_f_scheduler_post_fork (self=<optimized out>, arg0=0x35798f0) at src/cffi/native_engine.c:2234
        _save = 0x34a65a0
        x0 = 0x3740580
        datasize = <optimized out>
pantsbuild#14 0x00007f63f07b5a62 in PyEval_EvalFrameEx () from /lib64/libpython2.7.so.1.0
```

This presents as a hang in the thin client, because the pailgun socket is left open in the pantsd-runner.

### Solution

Add pre-fork hooks and tear down the `CpuPool` instances prior to forking and rebuilding them.

### Result

Can no longer reproduce the hang.