Change logging level of thrift plan in plan fragment executor.

VLOG(3) includes each row which is much less often useful than the serialized
plan.

Change-Id: I933188f046dafb51da9d06583697792113a9165a
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3289
Reviewed-by: Marcel Kornacker <marcel@cloudera.com>
Tested-by: jenkins