async notification patch for dblink

This patch adds the ability to retrieve async notifications using dblink,
via the addition of the function dblink_get_notify.

It is written against cvs head, includes documentation and regression
testing. It compiles, tests and works well.

I would be interested in some feedback on the regression test.
My initial thought was to test the function as thoroughly as possible. So I
perform listen and notify commands within the test to be able to test all
aspects of the code. Even though this works well for me, I get the feeling
that this is not the correct way to do it. I can find no other testing of
the listen/notify functionality in the regression tests, and I imagine this
is for good reason.
If someone would care to explain this, and maybe give a hint about what
amount of testing is appropriate for this fairly trivial patch, it would be
appreciated.

Best regards,

Marcus Kempe