gSoC add MERGE command new patch -- merge_v104

Hi,

Here comes the new patch for MERGE command. It has the following features:

1. It is based on Heikki's merge_v102-cleanedup.patch. So, it is (hopefully)
clean -- no meaningless white spaces and no overlong clause.

2. The "replaced" mark in MERGE query and plan structures are removed. In
rewriter, the actions replaced by INSTEAD rules will be changed into DO
NOTHING actions.

3. _outDeleteStmt() is removed from code.

4. EXPLAIN MERGE is improved much. You can see the new examples at
https://wiki.postgresql.org/wiki/MergeTestExamples#Explain_Merge

5. The subplan/sublinks are supported in merge actions now. Try the examples
at
https://wiki.postgresql.org/wiki/MergeTestExamples#Subplan.2Fsublinks_in_action

6. Updated merge.sql and merge.out for regress

7. The inheritance is still NOT supported yet.

Thanks

Regards

Yours Boxuan