Failover auth fix

This pull fixes an issue we've been experiencing in production because a replica set failover can trigger authentication failures. For these failures the Ignore strategy was used, which doesn't mark the node as down. Moped then keeps trying to authenticate with this node in down state which stalls the entire system.

In this pull the strategy for `AuthenticationFailure` is switched to `Retry` and we also mark a node as down when there's an authentication failure in the read operation. This behavior is similar to what was in place before 6f211acfdde53718490c01be928841d2705e794a. These changes (mostly) fix the issues we had during failover.