Changed active_rules in mesos-syle.py to array for easier management.

Review: https://reviews.apache.org/r/19743
Cleaned up a continuation in launcher.cpp.

Review: https://reviews.apache.org/r/19701
Updated RemoveSlave to use RepeatedPtrField::DeleteSubrange().

Review: https://reviews.apache.org/r/19712
Added a const qualifier to Stopwatch::elapsed.

Review: https://reviews.apache.org/r/19760