replication: unify replication filtering with and without elections

Once the split-brain detection is in place, it's fine to nopify obsolete
data even on a node with elections disabled. Let's not keep a bug around
anymore.

This behaviour change leads to changing
"gh_6842_qsync_applier_order_test.lua" a bit. It actually relied on old
and buggy behaviour: it assumed old transactions would not be nopified
and would trigger replication error.

This doesn't happen anymore, because nopify works correctly, and the
transactions are not followed by a conflicting CONFIRM.

The test for this commit is simply altering the
gh_5295_split_brain_detection_test.lua to work with elections disabled.

Closes #6133
Follow-up #5295

NO_DOC=internal change
NO_CHANGELOG=internal change