PYTHON-436 trying to work around Python 2.6 thread death issues with a monitor thread
PYTHON-436 gevent support doesn't need a monitor, the problem with
python 2.6 is with threads
PYTHON-436 when the monitor resets a request socket, set request state to NO_SOCKET_YET
PYTHON-436 filter out special sock_info values before checking them
PYTHON-436 fix the max_pool_size tests to work with the new behavior

The rendezvous is no longer needed as the max size of the pool is
tested elsewhere. These tests now simply make sure that no sockets are
leaked due to incorrect use of start_request/end_request by checking
the semaphore's value.