CDRIVER-4113: Thread-safe pool for server session objects (#847)

* CXX-4113: Thread-safe pool for server session objects

* Remove unused node pointers on server sessions

* Track pool size, and don't loop while holding the lock to clear

* Cleanup and some redesign of pool API

* Declare special pool for server sessions

* Mention the LIFO nature of the thread-pool, and the non-conformance of the 'push' operation

* auditing the pool is compile-time optional

* Remove -Waggregate-return