[SPARK-5529][CORE]Add expireDeadHosts in HeartbeatReceiver

If a blockManager has not send heartBeat more than 120s, BlockManagerMasterActor will remove it. But coarseGrainedSchedulerBackend can only remove executor after an DisassociatedEvent.  We should expireDeadHosts at HeartbeatReceiver.

Author: Hong Shen <hongshen@tencent.com>

Closes #4363 from shenh062326/my_change3 and squashes the following commits:

2c9a46a [Hong Shen] Change some code style.
1a042ff [Hong Shen] Change some code style.
2dc456e [Hong Shen] Change some code style.
d221493 [Hong Shen] Fix test failed
7448ac6 [Hong Shen] A minor change in sparkContext and heartbeatReceiver
b904aed [Hong Shen] Fix failed test
52725af [Hong Shen] Remove assert in SparkContext.killExecutors
5bedcb8 [Hong Shen] Remove assert in SparkContext.killExecutors
a858fb5 [Hong Shen] A minor change in HeartbeatReceiver
3e221d9 [Hong Shen] A minor change in HeartbeatReceiver
6bab7aa [Hong Shen] Change a code style.
07952f3 [Hong Shen] Change configs name and code style.
ce9257e [Hong Shen] Fix test failed
bccd515 [Hong Shen] Fix test failed
8e77408 [Hong Shen] Fix test failed
c1dfda1 [Hong Shen] Fix test failed
e197e20 [Hong Shen] Fix test failed
fb5df97 [Hong Shen] Remove ExpireDeadHosts in BlockManagerMessages
b5c0441 [Hong Shen] Remove expireDeadHosts in BlockManagerMasterActor
c922cb0 [Hong Shen] Add expireDeadHosts in HeartbeatReceiver