IMPALA-1110: group_concat agg function does not work with optional separator.

Rather than omit the first separator in each intermediate result,
always include the separator, but also remember the length of the
first separator.  Then, during finalize, remove whichever separator
string ends up at the beginning of the final merged result.

Change-Id: I6de7d1cda1a43b8de7d03c6798ec9667ffa457f8
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/3669
Reviewed-by: Marcel Kornacker <marcel@cloudera.com>
Tested-by: jenkins