Fixed slave to not recover completed executors.

Review: https://reviews.apache.org/r/13253
Added the ability to spoof the sender when posting a message.

Review: https://reviews.apache.org/r/13258
Fixed master to properly handle a disconnected slave.

Review: https://reviews.apache.org/r/13259
Clarified the guidance for users when slave recovery fails.

Review: https://reviews.apache.org/r/13261
Fixed master to properly reconcile KillTask messages.

Review: https://reviews.apache.org/r/13339