Rethink pg_dump's handling of object ACLs.

Rethink pg_dump's handling of object ACLs.

Throw away most of the existing logic for this, as it was very
inefficient thanks to expensive sub-selects executed to collect
ACL data that we very possibly would have no interest in dumping.
Reduce the ACL handling in the initial per-object-type queries
to be just collection of the catalog ACL fields, as it was
originally.  Fetch pg_init_privs data separately in a single
scan of that catalog, and do the merging calculations on the
client side.  Remove the separate code path used for pre-9.6
source servers; there is no good reason to treat them differently
from newer servers that happen to have empty pg_init_privs.

Discussion: https://postgr.es/m/2273648.1634764485@sss.pgh.pa.us
Discussion: https://postgr.es/m/7d7eb6128f40401d81b3b7a898b6b4de@W2012-02.nidsa.loc