This is a first stab at moving from SPI queries
to direct heap/index scans for cleaner and more
efficient code. By doing direct scans, there is no
need to prepare and cache a bunch of query plans
that make the code both slower and more complex.
This patch also adds a catalog module that keeps
cached OIDs and other things for catalog tables.
The cached information is updated every time the
backend switches to a new database. A permission
check is also implemented when accessing the catalog
information, but should probably be extended to
tables and schemas in the future.
