Fixed a file descriptor leak in StatusUpdateManager.

Review: https://reviews.apache.org/r/26879
Fixed MESOS-947: Slave should properly handle a killTask() that arrives
between runTask() and _runTask().
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