Temporarily disabled RunTaskWithCommandInfoWithUser due to MESOS-1359.
Fixed concurrent wait within the ExternalContainerizer.

The ECP must not receive multiple, concurrent wait-invocations on the
same ContainerID.

This fix checks if a wait-invocation is already active by checking if
the wait-PID is setup (isSome). If that is the case, the EC simply
returns the future of the termination without additionally invoking
the ECP.

Review: https://reviews.apache.org/r/21328
Ported some of the master stats to the new metrics library.

Review: https://reviews.apache.org/r/19504
Added message counters for all message types in Master.

Review: https://reviews.apache.org/r/21399
Updated Slave.MetricsInStatsEndpoint test to match latest metrics.

Review: https://reviews.apache.org/r/21417
Fixed a compiler warning from stout flags.

Review: https://reviews.apache.org/r/21462
Added task gauges to Master.

Review: https://reviews.apache.org/r/21279
Changed 'registry' flag default to 'replicated_log'.

Review: https://reviews.apache.org/r/21457
Fixed comment typos in a previous commit for message counters.
Added resource Gauges to Master.

Review: https://reviews.apache.org/r/21166