Socket leak in copy_database

Motor leaks one socket in each call to copy_database.

The socket is closed by garbage collection, but it continues to count toward max_pool_size. If max_pool_size is finite (the default is 100) then the client hangs after max_pool_size calls to copy_database.

Applies to MotorClient and MotorReplicaSetClient.

Motor users apparently don't call copy_database much, or this would have been reported earlier.