The client pool will try to hold on to live clients forever. If we have a watermark that we would like to stay below in the connection pool, we should destroy the oldest or unhealthiest connection.

This should likely happen on mongoc_client_pool_push() or potentially on a pop().

Ensure that minPoolSize is respected in the process. minPoolSize is lazy in terms of filling up to minPoolSize, but should not drop below after the connections have been made.