[Req] MERGE statement support

[ArsenShnurkov]

It is defined in the SQL:2003 standard.

quote from wikipedia:

MERGE INTO table_name USING table_reference ON (condition)
WHEN MATCHED THEN
UPDATE SET column1 = value1 [, column2 = value2 ...]
WHEN NOT MATCHED THEN
INSERT (column1 [, column2 ...]) VALUES (value1 [, value2 ...])

This statement (together with create sequence) is important to support modern programs and frameworks, ported from windows to mono.

This statement is syntax shugar, so it shouldn't be difficult to implement it.