Linq2db: More freedom on merge actions configuration

[MaceWindu]

Currently merge supports
WHEN MATCHED THEN UPDATE
WHEN NOT MATCHED BY TARGET THEN INSERT
WHEN NOT MATCHED BY SOURCE THEN DELETE

It will be more usefull, if type of operation could be specified by user.
E.g. in combination with #336 I would like to perform delete of matched records (by primary key and additional field, added as extra predicate).
Another example:
delete by match on composite primary key (no custom predicates required)

[ili]

What about PR? :)

[MaceWindu]

Not right now, but yes, I have it in plans for summer (design proposal for merge API and then implementation).

[MaceWindu]

Guys, I've started my work on this task and created some documentation. Feel free to add your comments/ideas:
https://github.com/MaceWindu/linq2db/wiki/MERGE-INTO-Command

[ili]

That's really cool.
I do have nothing to add :)
Awaiting for PR's )))