Add on_thread_exit hook

[biinari]

https://www.pivotaltracker.com/story/show/183131066

For apps that need to tidy up thread-local resources when a thread exits, add a new on_thread_exit hook.
This will be called when a thread is being trimmed, just before the thread exits.

An example config entry involving rabbitmq channels being held in thread local variables (one channel per thread):

on_thread_exit do
  unless Thread.current[MY_KEY].nil?
    Thread.current[MY_KEY].disconnect
    Thread.current[MY_KEY] = nil
  end
end

The hook should be configured with a quick non-blocking operation as it will block new requests from being started in other threads. I wondered about moving the call to the hook outside of the mutex section, but that becomes quite a bit less readable and I can't see what anyone would need to do that would be a blocking operation.