Updated CHANGELOG for 0.14.0.

Review: https://reviews.apache.org/r/13625

*** Modified for 0.13.0 *** Improved task validation error messages

in master.

Review: https://reviews.apache.org/r/13454

Revert "Fixed master to send a FrameworkReregistered message when the

framework re-registers with a failed over master."

This reverts commit 68ccbf1.

Conflicts:
	src/master/master.cpp
	src/tests/fault_tolerance_tests.cpp

Fixed slave to remove a queued task when the task is terminated.

Review: https://reviews.apache.org/r/13207

Updated CHANGELOG for 0.12.1.

Tag for 0.12.1-rc3.

Tag for 0.12.1-rc2.

Changed version to 0.12.1 in configure.ac.

Fixed issues from cherry-picks for 0.13.0-rc4.

Changed slave command line flag 'safe' to 'strict'.

Review: https://reviews.apache.org/r/12084