Instead of carrying a "new OID" field in all the structs that represent
CREATE statements, introduce a generic mechanism for capturing the OIDs
of all created objects, dispatching them to the QEs, and using those same
OIDs when the corresponding objects are created in the QEs. This allows
removing a lot of scattered changes in DDL command handling, that was
previously needed to ensure that objects are assigned the same OIDs in
all the nodes.

This also provides the groundwork for pg_upgrade to dictate the OIDs to use
for upgraded objects. The upstream has mechanisms for pg_upgrade to dictate
the OIDs for a few objects (relations and types, at least), but in GPDB,
we need to preserve the OIDs of almost all object types.