Split out MERGE and REPLACE/UPSERT items.

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