track waiters

[ThinkOpenly]

I'm not sure how easy this is, but it would be nice to report statistics on the number of tasks waiting for a lock, perhaps the maximum and average number of waiters, and elapsed and percent time that the number of waiters was greater than zero.
One significant challenge is accounting for "pending" states: determining how many waiters are active at the start of the trace. Impossible, actually, but some accounting can be done. If the first event for a task is mutex_acquire, we know it was waiting since the start of the trace.