Removing special case OID generation

Recent events have made me notice the OID handling.

AFAICS, OIDs are just a sequence with a max value that fits in a uint.

So ISTM that we should just strip out the OID handling code and just
have a system sequence defined like this

CREATE SEQUENCE _pg_oid
MINVALUE 0
MAXVALUE 4294967296
CACHE 8192
CYCLE;

Which would then make it easier to have a sequence for each toast
table and a sequence for lobs.

Not sure its important now, but maybe it will reduce the size of the
executable and avoid oid-specific bugs.

--
 Simon Riggs                   http://www.2ndQuadrant.com/
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