Revert "Allowed Gauge to take synchronous functions."

Bug: MESOS-1286

This reverts commit 249011b3d71fcc1776104132391c2011335788d5.
Added test for killing unknown task with Slaves in transitional state.

Review: https://reviews.apache.org/r/21249
Updated CHANGELOG for 0.18.2-rc1.
Introduced '--' to flags parsing.

Review: https://reviews.apache.org/r/21384
Added override of Flags::load that updates 'argc' and 'argv'.

Review: https://reviews.apache.org/r/21388
Added task gauges to slave metrics.

Review: https://reviews.apache.org/r/21276
Added support for passing flags in override command.

Calling the mesos executor with "./mesos-executor --override ./foo
--bar" would cause the executor flags parsing to fail as it would
continue evaluating arguments as potential flags candidates.  r21388
and r21384 adds the option to use a "--" terminator, which will cause
the mesos flag parsing to stop and modify argv to contain the
remaining arguments.

Review: https://reviews.apache.org/r/21408
mesos-style.py accepts files to lint as args.

Review: https://reviews.apache.org/r/21360
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
Fixed credentials::read() to properly stat the credentials file.

Review: https://reviews.apache.org/r/21445
Added API for managing links.

Review: https://reviews.apache.org/r/20292