Introduce a new lock_broken hook.

* tests/test_lockdir.py:
(TestLockDirHooks): Separate hook related tests from other lock
tests.
(TestLockDirHooks.test_LockDir_broken_success,
TestLockDirHooks.test_LockDir_broken_failure): Add tests for the
new lock_broken hook.

* lockdir.py:
(LockDir.force_break): Fire lock_broken hook.

* lock.py:
(LockHooks.__init__): Add lock_broken hook.