[2/2] spark git commit: SPARK-4682 [CORE] Consolidate various 'Clock' classes

SPARK-4682 [CORE] Consolidate various 'Clock' classes Another one from JoshRosen 's wish list. The first commit is much smaller and removes 2 of the 4 Clock classes. The second is much larger, necessary for consolidating the streaming one. I put together implementations in the way that seemed s

[2/2] spark git commit: SPARK-4682 [CORE] Consolidate various 'Clock' classes

SPARK-4682 [CORE] Consolidate various 'Clock' classes Another one from JoshRosen 's wish list. The first commit is much smaller and removes 2 of the 4 Clock classes. The second is much larger, necessary for consolidating the streaming one. I put together implementations in the way that seemed s