This bug may have been a regression introduced in optimizing the key removal: a7c3106

To reproduce refer to poc:
https://github.com/kevink-sq/jetty-concurrency-issue-poc

To verify fix, refer to poc branch (same as these changes):
https://github.com/kevink-sq/jetty-concurrency-issue-poc/tree/kevink/attempted-junixsocket-fix-2

Jetty's [ManagedSelector](https://github.com/jetty/jetty.project/blob/7a7d69a69f4f51772e20813332291189a24e91b1/jetty-io/src/main/java/org/eclipse/jetty/io/ManagedSelector.java#L655)
iterates through the selectedKeysSet and when separate processes cancel
the AFSelectorKey, the iterator throws a `ConcurrentModificationException`.

Fix is to introduce `cancelledKeysSet` to defer removal of cancelled
keys until the select process. Use of the cancelled sets are referenced
from java.nio.channels.spi's `AbstractSelector` and `AbstractSelectionKey`.

Ideally we'd have `AFSelectionKey` extend from `AbstractSelectionKey` to
leverage the base class's internal cancelled set
but`AbstractSelectionKey` defines its own `isValid` and `cancel` and
cannot override the base's final methods.

Also added an atomic check to  `AFSelectionKey#cancel` to prevent "Too
many open files" socket exception.

This reverts commit b8386a1.

Clients may iterate upon AFSelector.selectedKeys() while the selected
keys are modified in another thread.

Change the underlying datastructure to be thread-safe (use
ConcurrentHashMap), and add all "ready" selectors even if they're marked
invalid (they will be removed later).

We reimplement the fix as the original patch caused a FileDescriptor
leak and TIPC test failures.

#142
#145

Since we already have a ConcurrentHashMap, let's make use of the value,
which now indicates the "selected" state.

Introduce MapValueSet, which is a view over elements of the
"keysRegistered" map, and precisely only those elements that have a
certain value.

For each call to select, we increment the expected value, and then set
only the actually selected entries to that value, so we don't have to
clear the entire map.

#145

Fix errors only occurring with -Drelease

#145

In 2.9.0, we will a newer kohlschutter-parent.