pgsql: Split out MERGE and REPLACE/UPSERT items.

Log Message:
-----------
Split out MERGE and REPLACE/UPSERT items.

< * Add MERGE command that does UPDATE/DELETE, or on failure, INSERT (rules,
< triggers?)

* Add SQL-standard MERGE command, typically used to merge two tables

This is similar to UPDATE, then for unmatched rows, INSERT.
Whether concurrent access allows modifications which could cause
row loss is implementation independent.

* Add REPLACE or UPSERT command that does UPDATE, or on failure, INSERT

Modified Files:
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pgsql/doc:
TODO (r1.1706 -> r1.1707)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/TODO.diff?r1=1.1706&amp;r2=1.1707)
pgsql/doc/src/FAQ:
TODO.html (r1.213 -> r1.214)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/FAQ/TODO.html.diff?r1=1.213&amp;r2=1.214)