IMPALA-996: Exclude non-materialized slots from a tuple's avgSerializedSize.

Change-Id: Ic7936c6b5c5e6d4c162d91105128cda2b1b7284c
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2617
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins