Consolidated slave re-registration Timers into a single Timer.

Review: https://reviews.apache.org/r/19857
Added a configurable limit on the percentage of slaves that can be removed after the re-registration timeout.

Review: https://reviews.apache.org/r/20097
Added task reconciliation for unknown slaves and tasks.

Review: https://reviews.apache.org/r/20104
Updated killTask to only send TASK_LOST when a strict registry is in use.

Review: https://reviews.apache.org/r/20140
If a non-strict registry is in use, do not inform frameworks that slaves are lost if they do not re-register after a failover.

Review: https://reviews.apache.org/r/20383
Used LogStorage for all tests.

Review: https://reviews.apache.org/r/20431