HADOOP-10632. Minor improvements to Crypto input and output streams. Contributed by Yi Liu

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/fs-encryption@1598485 13f79535-47bb-0310-9956-ffa450edef68
HADOOP-10635. Add a method to CryptoCodec to generate SRNs for IV. Contributed by Yi Liu

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/fs-encryption@1598493 13f79535-47bb-0310-9956-ffa450edef68