https://issues.apache.org/jira/browse/SPARK-9202

Author: CodingCat <zhunansjtu@gmail.com>

Closes #7714 from CodingCat/SPARK-9202 and squashes the following commits:

23977fb [CodingCat] add comments about why we don't synchronize finishedExecutors & finishedDrivers
dc9772d [CodingCat] addressing the comments
e125241 [CodingCat] stylistic fix
80bfe52 [CodingCat] fix JsonProtocolSuite
d7d9485 [CodingCat] styistic fix and respect insert ordering
031755f [CodingCat] add license info & stylistic fix
c3b5361 [CodingCat] test cases and docs
c557b3a [CodingCat] applications are fine
9cac751 [CodingCat] application is fine...
ad87ed7 [CodingCat] trimFinishedExecutorsAndDrivers