[SPARK-6640][Core] Fix the race condition of creating HeartbeatReceiver and...

[SPARK-6640][Core] Fix the race condition of creating HeartbeatReceiver and retrieving HeartbeatReceiver

This PR moved the code of creating `HeartbeatReceiver` above the code of creating `schedulerBackend` to resolve the race condition.

Author: zsxwing <zsxwing@gmail.com>

Closes #5306 from zsxwing/SPARK-6640 and squashes the following commits:

840399d [zsxwing] Don't send TaskScheduler through Akka
a90616a [zsxwing] Fix docs
dd202c7 [zsxwing] Fix typo
d7c250d [zsxwing] Fix the race condition of creating HeartbeatReceiver and retrieving HeartbeatReceiver