Disable logical decoding as unsupported

Disable logical decoding as unsupported

Commit 665c224a6b2afa disabled CREATE PUBLICATION/SUBSCRIPTION, but
it was still possible to create a logical replication slot and call
pg_logical_slot_get_changes() on it.

That would however crash and burn as ReorderBufferCommit() relies on
subtransactions, and BeginInternalSubTransaction() is not expected
to fail, leading to segfaults in the PG_CATCH block.

Simply disallowing creating logical slots (and whatever else relies
on CheckLogicalDecodingRequirements) seems like the best fix.