Remove unused Interval.currentInterval() method.

This contained a reference to System.currentTimeMillis, so it probably ran
the risk of breaking our `Clock` abstraction.
Use Scalatest Assertions === in MasterFailureTest.
Add ability to block on batch start events.
WIP towards SPARK-1600
Synchronize `currentTime` in ManualClock
First passing version of the refactored SPARK-1600 test.