WAL bypass for CTAS

I enclose a complete patch for avoiding WAL usage for CREATE TABLE AS
SELECT, when not in archive mode (PITR). The main use case for this is
large BI environments that create summary tables or prejoined tables,
though there are many general applications.

There is no user interface for this. The speed gain is automatic, when
archiving is not enabled.

This contains all the lower level machinery required to do the same
thing for COPY, as discussed on hackers. The machinery includes some
additional freespace thinkery, aimed mainly at the forthcoming COPY
patch, which solely needs to be integrated with Alon's work.

Patch is diff -c format, compiles and make checks on cvstip as of now.

No performance tests *on this patch*, though the general principle has
already been proven via a similar prototype patch not published on list.

Best Regards, Simon Riggs