Re: background sessions

Here is a patch for the background sessions C API and PL/Python support.
 This was previously submitted as "autonomous transactions", which
proved controversial, and there have been several suggestions for a new
name.

I have renamed everything, removed all the incomplete PL/pgSQL stuff,
did some refinement on the PL/Python interfaces, added resource owner
management so that you can preserve session handles across transactions.
 That allows a pg_background-like behavior implemented in a PL function.
 I have also added documentation, so reviewers could start there.
Probably not quite all done yet, but I think it contains a lot of useful
pieces that we could make into something nice.