Replace hardcoded 200000000 with autovacuum_freeze_max_age

Parts of the code used autovacuum_freeze_max_age to determine whether
anti-multixact-wraparound vacuums are necessary, while others used a
hardcoded 200000000 value.  This leads to problems when
autovacuum_freeze_max_age is set to a non-default value.  Use the latter
everywhere.

Backpatch to 9.3, where vacuuming of multixacts was introduced.

Andres Freund