make known_columns composite keys deferrable and defer during upgrades
make known_columns composite keys deferrable and defer during upgrades

Since we may detect changing OIDs and column ordinal positions when
upgrading a catalog which has been dumped/reloaded, we need to defer
the composite key tests which might collide temporarily as rows are
mutated in the `_ermrest.known_columns` table during resynchronization
of the stored model by name.

We don't normally expect changes in ordinal position and OID during
operation, so we only defer during this offline script.  A DBA might
also do the same thing if they encounter trouble with a manual
`model_change_event()` invocation.

informatics-isi-edu/ermrest,informatics-isi-edu/ermrest