locking: Implement priorities in SharedLock and LockSet

For proper support of job priorities, jobs' locks need to respect
priorities.  Otherwise it could happen that a job with a lower priority
could get a lock before a job with a higher priority (depending on
timeouts and when they start acquiring).

This patch adds support for priorities in SharedLock and LockSet and
provides (unfortunately non-trivial) unittests. Outdated comments are also
adjusted and improved.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>