test__threadpool.py: use built-in sleep() instead of gevent.sleep() in the pool, because otherwise a new Hub is created and screws up reference count checks

ThreadPool: fix _set_size to stop _fork_watcher if it's not needed; also make sure it does not try to sleep when run inside hub

This is to counter cases where SIGCHLD is not delivered or lost which I've seen happening with test_patched_signal.py

fileobject: use __del__ implemented in Cython so that it cannot be interrupted asynchronously; multiple bugfixes

 - add 'close' argument to SocketAdapter
 - handle EINTR
 - remove _flushlock
 - remove closedfileobject