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box: disable split-brain detection until schema is upgraded

Our split-brain detection machinery relies among other things on all
nodes tracking the synchro queue confirmed lsn. This tracking was only
added together with the split-brain detection. Only the synchro queue
owner tracked the confirmed lsn before.

This means that after an upgrade all the replicas remember the latest
confirmed lsn as 0, and any PROMOTE/DEMOTE request from the queue owner
is treated as a split brain.

Let's fix this and only enable split-brain detection on the replica set
once the schema version is updated. Thanks to the synchro queue freeze
on restart, this can only happen after a new PROMOTE or DEMOTE entry is
written by one of the nodes, and thus the correct confirmed lsn
is propagated with this PROMOTE/DEMOTE to all the cluster members.

Closes #8996

NO_DOC=bugfix
NO_TEST=2.10 lacks box.schema.downgrade(), which's used in the test

(cherry picked from commit a844bd37a)

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