Allow failover for replicas

[magec]

Now that #847 is merged, when a pool is set to default_role = replica (or manually set to replica), and no replicas are available, the primary won't be touched at all. Instead, replicas will be automatically set back to available and will be checked for health again.

If you have a single replica and it is actually down, you may find convenient to send queries to the primary until you fix the replica (if the primary can handle the load). To allow this behavior, a new configuration parameter is added to the configuration that will make the primary available to receive queries if every replica in the pool is banned.

### replica_to_primary_failover_enabled
` ` `
path: pools.<pool_name>.replica_to_primary_failover_enabled
default: "false"
` ` ` 

If set to true, when the specified role is `replica` (either by setting `default_role` or manually)
and all replicas are banned, queries will be sent to the primary (until a replica is back online).

Note that when this is enabled, the replicas won't be automatically unbanned (default behavior).