Removing the use of 'null' in LARS
fix yet another bug in LARS, see #3471

- the scheduler should round up the calculated slot instead of rounding
  down
- SchedulerSpec did not catch this because it added 5ms leeway (which is
  wrong and now removed)
- InboxActor exposed it by being really picky with early-firing timers;
  change that to be more robust
Merge pull request #1570 from akka/wip-the-great-un-nulling-√

Removing the use of 'null' in LARS
Merge remote-tracking branch 'upstream/master'