MasterSlaveConnection.in_request() and MongoReplicaSetClient.in_request() are wrong if multithreaded

Repro:

Create MasterSlaveConnection or MongoReplicaSetClient with auto_start_request=False. Thread A calls start_request(). Thread B calls in_request().

Expected: in_request() is False for thread B

Actual: in_request() is True for thread B, but thread B is not actually in a request (it doesn't have a reserved socket on master / primary)