Improvements to the application pause monitor.

- Use System.nanoTime() to remove false-positive from network time adjustments
  and other external factors.
- Don't track total block time, It isn't a useful metric and it was confusing.
- Have the "trigger" level be the max block time that's acceptable, rather than
  the max block time plus the monitor pause interval.
- Bump the default block time trigger level to 200ms
- Introduce a manual mechanism to verify that the monitor works.
Merge pull request #3076 from jakewins/2.0-gc-logging

Improvements to the application pause monitor.