[SPARK-6602][Core] Replace direct use of Akka with Spark RPC interface - part 1

This PR replaced the following `Actor`s to `RpcEndpoint`:

1. HeartbeatReceiver
1. ExecutorActor
1. BlockManagerMasterActor
1. BlockManagerSlaveActor
1. CoarseGrainedExecutorBackend and subclasses
1. CoarseGrainedSchedulerBackend.DriverActor

This is the first PR. I will split the work of SPARK-6602 to several PRs for code review.

Author: zsxwing <zsxwing@gmail.com>

Closes #5268 from zsxwing/rpc-rewrite and squashes the following commits:

287e9f8 [zsxwing] Fix the code style
26c56b7 [zsxwing] Merge branch 'master' into rpc-rewrite
9cc825a [zsxwing] Rmove setupThreadSafeEndpoint and add ThreadSafeRpcEndpoint
30a9036 [zsxwing] Make self return null after stopping RpcEndpointRef; fix docs and error messages
705245d [zsxwing] Fix some bugs after rebasing the changes on the master
003cf80 [zsxwing] Update CoarseGrainedExecutorBackend and CoarseGrainedSchedulerBackend to use RpcEndpoint
7d0e6dc [zsxwing] Update BlockManagerSlaveActor to use RpcEndpoint
f5d6543 [zsxwing] Update BlockManagerMaster to use RpcEndpoint
30e3f9f [zsxwing] Update ExecutorActor to use RpcEndpoint
478b443 [zsxwing] Update HeartbeatReceiver to use RpcEndpoint