Concurrent psql patch

Fixed the major omissions that made it incomplete.

- Added sgml documentation and \? usage
- Added basic mvcc regression tests using new functionality
- Fixed cursor-mode (FETCH_COUNT) functionality
- Removed \cwait in favour of psql variable ASYNC_DELAY

I'm still not sure it's quite polished enough to commit but if there's any
feedback I'll be happy to fix up anything that's not acceptable.

Also, if anyone has any better ideas for names than \cswitch and \cnowait
now's the time. I had intended them only as placeholders because I couldn't
think of anything better but it doesn't sound like anyone else has any better
ideas either. If not then we're going to be stuck with them. More or less,
it's explicitly described as an "experimental" feature in the docs so I
suppose we could always change them later.