Update backend to support hdfs caching.

Change-Id: I22761c8893c8fd222564d4e2a97bfba1284cd741
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1721
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: Nong Li <nong@cloudera.com>
Add BytesWritten counter.

Change-Id: If31f447a9dd38a9932b78a527c31f983cae749bd
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1711
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins