IMPALA-866: Make HdfsScanNode.computeStats() idempotent with respect to totalBytes_.

Change-Id: I1c243b089db82c0544586a2a1428081aa2dbcd52
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1844
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins