Forked processes are prone to re-using a previous socket, which can lead to all sorts of IO errors (e.g. out-of-order replies on the wire protocol) between the parent and child processes.
The legacy driver annotated persisted sockets with a PID so that they could be forgotten during selection if the PID did not still match.
PHPC-433, we should include the process ID in the mongoc_client_t hash to ensure that persisted clients are not reused if a forked process constructs a new Manager. We should also document that Managers should not be created before forking and used in child processes.