Added a test for killing an unknown task.

Review: https://reviews.apache.org/r/21179
Minor fix in post-reviews.
ProcTest.MultipleThreads waits for /proc to update.

Review: https://reviews.apache.org/r/21122
Ported some of the slave statistics to the new metrics library.

Review: https://reviews.apache.org/r/19499
Added intersection test for stout Interval and IntervalSet.

Review: https://reviews.apache.org/r/21190
Added equal operator for stout Interval.

Review: https://reviews.apache.org/r/21237
Implemented a basic Authorizer interface.

Review: https://reviews.apache.org/r/18730