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.

(cherry picked from commit c5828c624f37f78dad46c66cb878b392e4a1495c)