pgsql: Don't allow relminmxid to go backwards during VACUUM FULL

Don't allow relminmxid to go backwards during VACUUM FULL

We were allowing a table's pg_class.relminmxid value to move backwards
when heaps were swapped by VACUUM FULL or CLUSTER.  There is a
similar protection against relfrozenxid going backwards, which we
neglected to clone when the multixact stuff was rejiggered by commit
0ac5ad5134f276.

Backpatch to 9.3, where relminmxid was introduced.

As reported by Heikki in
http://www.postgresql.org/message-id/52401AEA.9000608@vmware.com

Branch
------
REL9_4_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/9eecc8a7cafa2be356a859f74c5db5961e190579

Modified Files
--------------
src/backend/commands/cluster.c |    6 ++++++
1 file changed, 6 insertions(+)