report amount of time that a locker blocks other acquirers

Currently we report how long threads waited to acquire locks, but it is harder to find the reason why the lock wasn't granted. By adding statistics on how long a locker holds a lock while blocking other threads, we could surface problematic lockers.

This should be straightforward to implement, because we already have the appropriate lock manager bucket locked when queuing the request, so we can just add a start block time to the granted request.