Implement an execption handling mechanism that can be injected in ThreadWithException.

* bzrlib/tests/test_test_server.py:
(TestTCPServerInAThread.test_exception_swallowed_while_serving):
Check that the expected exception is swallowed.

* bzrlib/tests/test_server.py:
(ThreadWithException.set_ignored_exceptions): Allows exception
handlers to be installed.
(ThreadWithException.join): Respect ignored_exceptions.
(TestingTCPServerMixin.set_ignored_exceptions)
(TestingTCPServerMixin._pending_exections): New helpers.
(TestingThreadingTCPServer.process_request)
(TestingThreadingTCPServer.set_ignored_exceptions): Propagate
exception_handler.
(TestingTCPServerInAThread.set_ignored_exceptions): New helper.