[SPARK-5124][Core] A standard RPC interface and an Akka implementation

This PR added a standard internal RPC interface for Spark and an Akka implementation. See [the design document](https://issues.apache.org/jira/secure/attachment/12698710/Pluggable%20RPC%20-%20draft%202.pdf) for more details.

I will split the whole work into multiple PRs to make it easier for code review. This is the first PR and avoid to touch too many files.

Author: zsxwing <zsxwing@gmail.com>

Closes #4588 from zsxwing/rpc-part1 and squashes the following commits:

fe3df4c [zsxwing] Move registerEndpoint and use actorSystem.dispatcher in asyncSetupEndpointRefByURI
f6f3287 [zsxwing] Remove RpcEndpointRef.toURI
8bd1097 [zsxwing] Fix docs and the code style
f459380 [zsxwing] Add RpcAddress.fromURI and rename urls to uris
b221398 [zsxwing] Move send methods above ask methods
15cfd7b [zsxwing] Merge branch 'master' into rpc-part1
9ffa997 [zsxwing] Fix MiMa tests
78a1733 [zsxwing] Merge remote-tracking branch 'origin/master' into rpc-part1
385b9c3 [zsxwing] Fix the code style and add docs
2cc3f78 [zsxwing] Add an asynchronous version of setupEndpointRefByUrl
e8dfec3 [zsxwing] Remove 'sendWithReply(message: Any, sender: RpcEndpointRef): Unit'
08564ae [zsxwing] Add RpcEnvFactory to create RpcEnv
e5df4ca [zsxwing] Handle AkkaFailure(e) in Actor
ec7c5b0 [zsxwing] Fix docs
7fc95e1 [zsxwing] Implement askWithReply in RpcEndpointRef
9288406 [zsxwing] Document thread-safety for setupThreadSafeEndpoint
3007c09 [zsxwing] Move setupDriverEndpointRef to RpcUtils and rename to makeDriverRef
c425022 [zsxwing] Fix the code style
5f87700 [zsxwing] Move the logical of processing message to a private function
3e56123 [zsxwing] Use lazy to eliminate CountDownLatch
07f128f [zsxwing] Remove ActionScheduler.scala
4d34191 [zsxwing] Remove scheduler from RpcEnv
7cdd95e [zsxwing] Add docs for RpcEnv
51e6667 [zsxwing] Add 'sender' to RpcCallContext and rename the parameter of receiveAndReply to 'context'
ffc1280 [zsxwing] Rename 'fail' to 'sendFailure' and other minor code style changes
28e6d0f [zsxwing] Add onXXX for network events and remove the companion objects of network events
3751c97 [zsxwing] Rename RpcResponse to RpcCallContext
fe7d1ff [zsxwing] Add explicit reply in rpc
7b9e0c9 [zsxwing] Fix the indentation
04a106e [zsxwing] Remove NopCancellable and add a const NOP in object SettableCancellable
2a579f4 [zsxwing] Remove RpcEnv.systemName
155b987 [zsxwing] Change newURI to uriOf and add some comments
45b2317 [zsxwing] A standard RPC interface and An Akka implementation