PHPC-1722: Destroy persistent clients in GSHUTDOWN
PHPC-1722: Destroy persistent clients in GSHUTDOWN

This ensures persistent clients in all threads are freed, not just those in the main thread. Doing so also required that we call mongoc_cleanup and bson_mem_restore_vtable in the final GSHUTDOWN (tracked by an atomic counter).

mongodb-labs/mongo-php-driver-prototype,mongodb/mongo-php-driver,mongodb-labs/mongo-php-driver-prototype,mongodb/mongo-php-driver,mongodb-labs/mongo-php-driver-prototype,mongodb/mongo-php-driver,mongodb-labs/mongo-php-driver-prototype,jmikola/mongo-php-driver,mongodb-labs/mongo-php-driver-prototype,jmikola/mongo-php-driver,jmikola/mongo-php-driver,mongodb/mongo-php-driver,mongodb/mongo-php-driver,jmikola/mongo-php-driver,jmikola/mongo-php-driver