On SDKs which use protobufs, the engine has objects compiled against a specific
version of protobuf. Normally this is fine, we take care on Linux to use the
same C++ ABI. On macOS however, we use libc++ to enable C++11 functionality,
whereas the protobuf library has been compiled with libstc++. These ABIs are
not compatible.

To address the problem, we introduce PbHandle. PbHandle is a wrapper around
protobuf::Message with two added pieces of state: whether or not the handle
"owns" the message (and can free it in its destructor), and whether or not
the handle was created by the engine (private) or created by SourceMod
(local).

Whenever we transfer a protobuf::Message pointer to SourceMod, we must take
care to convert it to a Local version first. Whenever we transfer a protobuf
pointer to the engine, we must convert it to a Private handle.

For platforms with no ABI differences (almost all of them), the handle is a
no-op. The private and local localities are compatible and no translation
takes place.

On macOS, CS:GO does require translation. SourceMod loads a tiny shim
library that contains a copy of the protobuf sources compiled against the
game's ABI. It then provides serialization and deserialization methods.
SourceMod must not interact with the game's protobuf objects without first
going through this proxy library.

Note that PbHandle is not quite like unique_ptr. It can be converted into a
PbHandle that does not destroy the underlying object. This is mainly because
UserMessages.cpp has rather complex state, so it is useful to track locality
without destroying an object. An unowned PbHandle must not outlive the
owning PbHandle.

This will fix a utf8 regression reported in issue #1286.

When creating our own "owned and local" protobuf message in `StartProtobufMessage`, `m_FakeEngineBuffer` is used to track that message. In `EndMessage` the message is optionally converted to a "private" one with the right abi on osx and passed to the engine's `SendUserMessage`. On linux and windows the same message as in the `m_FakeEngineBuffer` is passed though without conversion. `engine->SendUserMessage` has a vtable hook which sets `m_FakeEngineBuffer` to the passed argument.

`m_FakeEngineBuffer` frees the message it previously held, since it's "owned" from `StartProtobufMessage`, but that's the same one that's passed in as argument so a use-after-free in the engine happens when the now-freed message pointer is forwarded to the real `SendUserMessage` in the engine.

The message created in `StartProtobufMessage` wasn't free'd at all when hooks are blocked too. This fix moves the message buffer into a local variable which is destroyed at the end of the function.

Fixes #1286 and #1296