If a long-lived lock is stale and has no alive file,
LockFile::try_acquire will never succeed to acquire the lock. Fix this
by creating the alive file for all lock types and making
LockFile::try_acquire exit when lock activity is seen instead of
immediately after failing to acquire the lock.

Another advantage is that a stale lock can now always be broken right
away if the alive file exists.

Instead of running one keep-alive thread per lock, a long-lived LockFile
now lets a separate LongLivedLockFileManager object handle keep-alive
for several locks in a single thread.

This was kind of due to a typo in
0babd33e84147e923a729ee07a3b85097ec8baa8. Since the LongLivedLockFile
class is not used yet, the bug does not affect any released code.