The changes I made in 578b2297 assigned oids below
FirstBootstrapObjectId to objects in include/catalog/*.dat files that
did not have an oid assigned, starting at the max oid explicitly
assigned.  Tom criticized that for mainly two reasons:
1) It's not clear which values are manually and which explicitly
   assigned.
2) The space below FirstBootstrapObjectId gets pretty crowded, and
   some PostgreSQL forks have used oids >= 9000 for their own objects,
   to avoid conflicting.

Thus create a new range for objects not assigned explicit oids, but
assigned by genbki.pl. For now 1-9999 is for explicitly assigned oids,
FirstGenbkiObjectId (10000) to FirstBootstrapObjectId (1200) -1 is for
genbki.pl assigned oids, and < FirstNormalObjectId (16384) is for oids
assigned during bootstrap.  It's possible that we'll have to adjust
these boundaries, but there's some headroom for now.

Add a note suggesting that oids in forks should be assigned in the
9000-9999 range.

Catversion bump for obvious reasons.

Per complaint from Tom Lane.

Author: Andres Freund
Discussion: https://postgr.es/m/16845.1544393682@sss.pgh.pa.us