[SPARK-5214][Core] Add EventLoop and change DAGScheduler to an EventLoop

This PR adds a simple `EventLoop` and use it to replace Actor in DAGScheduler. `EventLoop` is a general class to support that posting events in multiple threads and handling events in a single event thread.

Author: zsxwing <zsxwing@gmail.com>

Closes #4016 from zsxwing/event-loop and squashes the following commits:

aefa1ce [zsxwing] Add protected to on*** methods
5cfac83 [zsxwing] Remove null check of eventProcessLoop
dba35b2 [zsxwing] Add a test that onReceive swallows InterruptException
460f7b3 [zsxwing] Use volatile instead of Atomic things in unit tests
227bf33 [zsxwing] Add a stop flag and some tests
37f79c6 [zsxwing] Fix docs
55fb6f6 [zsxwing] Add private[spark] to EventLoop
1f73eac [zsxwing] Fix the import order
3b2e59c [zsxwing] Add EventLoop and change DAGScheduler to an EventLoop