Allow to close thread dispatcher

[Menduist]

When a thread finishes, there is currently no way to release the resources of the dispatcher. Which on linux, include a FD & relatively big buffer for epoll

We could use onThreadDestruction to call "close" automatically, but that raises questions in what to do with pending futures

[cheatfate]

This is definitely not enough, number of callbacks could zero right after poll() call but it doesn't mean that there is no pending socket events, or pending timers. And even in such case you should check if this callback is actually SentinelCallback. Call to closeThreadDispatcher could be made while main poll() loop processing callbacks, so its possible to get into situation when callbacks <= 1, but even in such case its possible to get into situation when there no SentinelCallback.

[Menduist]

Good point, I'll check that this is the sentinel, and that it's not called from inside poll

[cheatfate]

Closing IOCP port or EPOLL/KQUEUE file descriptor do not closes all the files/sockets/pipes which was registered with it. So with this PR the only resource you are cleaning is IOCP port or EPOLL/KQUEUE file descriptor - nothing more. Your PR do not even check if there some descriptors registered in IOCP/EPOLL/KQUEUE.

[Menduist]

I would argue that the sockets the user opened are to be closed by the user

We can imagine that a user does something where he opens a socket on one thread, and then switch its processing to another thread, so we shouldn't close it if he stops the first thread

[cheatfate]

Of course, but i think the main idea of calling poll() in loop is to allow thread to get into state when there is no more unreleased resources except IOCP and KQUEUE/EPOLL data structure and descriptor.