SPARK-3093 : masterLock in Worker is no longer need

there's no need to use masterLock in Worker now since all communications are within Akka actor

Author: CrazyJvm <crazyjvm@gmail.com>

Closes #2008 from CrazyJvm/no-need-master-lock and squashes the following commits:

dd39e20 [CrazyJvm] fix format
58e7fa5 [CrazyJvm] there's no need to use masterLock now since all communications are within Akka actor
[SPARK-2862] histogram method fails on some choices of bucketCount

Author: Chandan Kumar <chandan.kumar@imaginea.com>

Closes #1787 from nrchandan/spark-2862 and squashes the following commits:

a76bbf6 [Chandan Kumar] [SPARK-2862] Fix for a broken test case and add new test cases
4211eea [Chandan Kumar] [SPARK-2862] Add Scala bug id
13854f1 [Chandan Kumar] [SPARK-2862] Use shorthand range notation to avoid Scala bug