SAMZA-163: Remove mutable.ConcurrentMap when we stop supporting 2.9
Fix typo in coding guide

Signed-off-by: Jakob Homan <jghoman@gmail.com>
SAMZA-226; auto-create changelogs
Merge branch 'master' into SAMZA-468
remove TODO
add docs to negate number task
aggregate YARN logs
remove HDFS dependency in samza-test, since we don't use it. fix ZK dependency version in YARN to be compatible with Kafka's ZK version.