Move semaphore unlink() from Base to WithLockMixin
Reset lock when creating a new base or KtbsRoot

This solves the following problem:
1. we delete a base B1
2. our kTBS crashes and somehow a lock on B1 is still acquired
3. we launch our kTBS
4. we create B1
5. we do somehting on B1 that requires the lock B1

Before this commit, we would wait and break on step (5.) because eventhough the base B1 doesn't exist, its lock is locked.

Now, when we create B1, we reset its lock, so step (5.) is ok.