Fixed memory leaks and cleaned up the mesos containerizer logic.

Review: https://reviews.apache.org/r/22159
Updated dropped message logging to use VLOG.

Review: https://reviews.apache.org/r/22164
Added a concurrent 'Queue' to libprocess.

Review: https://reviews.apache.org/r/20308
Introduced a low-level scheduler API.

Review: https://reviews.apache.org/r/20309