HADOOP-10681. Remove unnecessary synchronization from Snappy & Zlib codecs. Contributed by Gopal Vijayaraghavan.

(cherry picked from commit 8f9ab998e273259c1e7a3ed53ba37d767e02b6bb)
HADOOP-10731. Sync-ing CHANGES.txt across branches.
Merge remote-tracking branch 'remotes/origin/branch-2' into branch-2