Added 'metadata' to the replicated log to support log recovery (and
likely in the future log reconfiguration).

From: Jie Yu <yujie.jay@gmail.com>
Review: https://reviews.apache.org/r/15799
*** Modified for 0.15.0 *** Drop messages from non-leading Masters in the scheduler driver.

Review: https://reviews.apache.org/r/15778

Conflicts:
	src/sched/sched.cpp
*** Modified for 0.15.0 *** Fixed master to ignore messages from unregistered framework.

Review: https://reviews.apache.org/r/15773

Conflicts:
	src/master/master.cpp
Updated CHANGELOG for 0.15.0 bug fixes.
Merge tag '0.15.0' into debian-package-0.15.0
Update to 0.15.0 release