Offshoot of PHPC-1274:

In order to avoid calling mongoc_client_reset() redundantly, the PHP driver should include some logic to determine if it has already been invoked for a given PID.

This can be done by allocating a hash table in GINIT (so it is reset for each process and thread) to track which clients have been reset. The various free_object handlers modified in PHPC-1274 can then consult this hash table before calling mongoc_client_reset() to avoid advancing the mongo_client_t's generation counter more times than necessary.