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
Consolidated "launch" code paths in Docker containerizer.

Review: https://reviews.apache.org/r/26610
Allow docker containers to be scheduled for removal
Fix docker flaky tests

Conflicts:
	src/slave/containerizer/docker.cpp
Add priviledged option to docker info
Support lxc configurations in docker containerizer
Add docker hostname option.
Symlink sandbox directories in docker containerizer

Review: https://reviews.apache.org/r/26517
Avoid docker inspect on each usage call

Review: https://reviews.apache.org/r/26436