Respect failover disabled config option with per_repository elector

Praefect has a config option toggling whether the failover logic is enabled or disabled. When repository-specific primaries are enabled, the configuration option is not currently respected. This commit fixes the situation by not running the election loop in case failover is disabled. Rest of the logic stays the same, and the primaries are still being read from the database, unlike with sql elector with failover disabled. Praefect just doesn't update the primary records if failovers are disabled.

Related to: #3611 (closed)