track pending time

[ThinkOpenly]

If the first event for a lock is acquired, the time from the start of the trace to then was spent waiting to acquire the lock, but we don't know the entire amount of time spent waiting. This time should be accumulated in a "pending wait" bucket and reported.

Similarly for release (pending held time).

Also, if the last event for a lock is acquired, the time from the event to the end of the trace should be accounted for as "pending held" time.

Similarly for entry (pending wait time).