Spawn a thread for each connection from a client.

* bzrlib/tests/test_test_server.py:
(TestTestingServerInAThread.test_client_talks_server_respond):
Check that the connection has been collected.

* bzrlib/tests/test_server.py:
(TestingTCPServerMixin.get_request)
(TestingThreadingTCPServer.get_request): Collect the resources
used for a client connection.
(TestingThreadingTCPServer.process_request): Spawn a thread for
each connection.