Collect and shutdown clients for SmartTCPServer_for_testing.

* bzrlib/tests/http_server.py:
(TestingHTTPServerMixin.serve): Try to avoid hanging in the server
thread, we already catch the socket.timeout exception, we may as
well use it.
(TestingThreadingHTTPServer.process_request_thread): Get rid of
the spurious pdb.set_trace().

* bzrlib/tests/__init__.py:
(ExtendedTestResult.stopTestRun): Display how many threads are
left at the end.
(TestCase._check_leaked_threads): 

* bzrlib/smart/server.py:
(SmartTCPServer.serve_conn): Return the created thread so daughter
classes can play with it.
(SmartTCPServer_for_testing.serve_conn): Collect the client
sockets/threads.
(SmartTCPServer_for_testing.shutdown_client): shutdown a client
socket.
(SmartTCPServer_for_testing.tearDown): shutdown client
sockets/threads.