Split authorization from task validation.

Review: https://reviews.apache.org/r/26817
Add dispatch event counters to Scheduler driver

Review: https://reviews.apache.org/r/26951
Added a document for reconciliation.

Review: https://reviews.apache.org/r/26669
Added reconciliation to the documentation home.

Review: https://reviews.apache.org/r/26670
Updated status update manager to forward updates via slave.

Review: https://reviews.apache.org/r/26846
Added pause() and resume() to status update manager.

Review: https://reviews.apache.org/r/26957
Updated slave re-registration to send unacknowledged task states.

Review: https://reviews.apache.org/r/26699
Updated slave to include latest task state in update.

Review: https://reviews.apache.org/r/26700
Updated master to update task unacknowledged state properly.

Review: https://reviews.apache.org/r/26701
Updated reconciliation semantics to take the task's unacknowledged state into account.

Review: https://reviews.apache.org/r/26702