config: add replication.failover option

It has the only one value for now: 'off'. Two other ones are planned:
'manual' and 'election'.

The 'off' value means exactly what is implemented now: the database.mode
option controls whether a particular instance should serve write
requests.

Part of #8810

NO_DOC=the old behavior was not released, the documentation request will
       be registered manually
NO_CHANGELOG=see NO_DOC