Hot standby, slot ids and stuff

I've mentioned before that I don't like the slotid stuff. From an
architectural point of view, we should try to keep the extra
communication between primary and standby to a minimum, for the sake of
robustness. The primary shouldn't have such a big role in keeping track
of which xids it has already emitted in WAL records, which subxids have
been marked, and so on.

Since I've been whining about that for some time, I figured I have to
put my money where my mouth is, so here's a patch based on v6a that
eliminates the concept of slotids, as well as the new xl_info2 field in
XLogRecord. This seems much simpler to me. I haven't given it much
testing, but seems to work. There's a whole bunch of comments marked
with XXX that need resolving, though.

Attached is a patch agains CVS HEAD (hs-noslotid-1.patch) as well as an
incremental patch against your v6a (hs-noslotid-against-v6a.patch).
Sorry if you were just working on some big changes and I joggled your
elbow. I also didn't check what changes you had in v7 yet.

--
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com