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