Support "Right Semi Join" plan shapes

In thread [1] which discussed 'Right Anti Join', Tom once mentioned
'Right Semi Join'.  After a preliminary investigation I think it is
beneficial and can be implemented with very short change.  With 'Right
Semi Join', what we want to do is to just have the first match for each
inner tuple.  For HashJoin, after scanning the hash bucket for matches
to current outer, we just need to check whether the inner tuple has been
set match and skip it if so.  For MergeJoin, we can do it by avoiding
restoring inner scan to the marked tuple in EXEC_MJ_TESTOUTER, in the
case when new outer tuple == marked tuple.

As that thread is already too long, fork a new thread and attach a patch
used for discussion.  The patch implements 'Right Semi Join' for
HashJoin.

[1] https://www.postgresql.org/message-id/CAMbWs4_eChX1bN%3Dvj0Uzg_7iz9Uivan%2BWjjor-X87L-V27A%2Brw%40mail.gmail.com

Thanks
Richard