HDFS-5266. ElasticByteBufferPool#Key does not implement equals. Contributed by Chris Nauroth.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/HDFS-4949@1526671 13f79535-47bb-0310-9956-ffa450edef68
HDFS-5266: addendum, add null check in equals and add hashCode
merge HDFS-4949 to HDFS-5266