PHPC-912 introduced a php_phongo_pclient_t struct to encapsulate both the mongoc_client_t and PID, which is persisted across requests. When the persistent hash's destroy function was changed to only free mongoc_client_t if the PID matched the current process, we neglected to add a free on the php_phongo_pclient_t wrapper, which should always happen.

This is not a major issue, since the memory is leaked during MSHUTDOWN.

Note: this was discovered while checking for outstanding PHP 7 memory leaks for PHPC-500.