Add `total_delay` to handler arguments

Pass the total time waited to the handler.  This is to facilitate, for example,
bumping a stat or emitting a warning when delays exceed a threshold.
Further tweaks to `total_delay` handler arg

- Improve method comment
- Tidy up test
 - Speed up test
Merge remote-tracking branch 'mmazour/add-total_delay'
Clean up total_delay test a little.

- Use RR to mock out Time.now to make test timing-independent and ~100x faster.
- Remove unused variable