[SPARK-4005][CORE] handle message replies in receive instead of in the individual private methods

In BlockManagermasterActor, when handling message type UpdateBlockInfo, the message replies is in handled in individual private methods, should handle it in receive of Akka.

Author: Zhang, Liye <liye.zhang@intel.com>

Closes #2853 from liyezhang556520/akkaRecv and squashes the following commits:

9b06f0a [Zhang, Liye] remove the unreachable code
bf518cd [Zhang, Liye] change the indent
242166b [Zhang, Liye] modified accroding to the comments
d4b929b [Zhang, Liye] [SPARK-4005][CORE] handle message replies in receive instead of in the individual private methods
[SPARK-4765] Make GC time always shown in UI.

This commit removes the GC time for each task from the set of
optional, additional metrics, and instead always shows it for
each task.