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
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master

Details
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http://git.postgresql.org/pg/commitdiff/b7e51d9c06e6a0da50abbbd0603ecb80f0b6f02b

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