Cluster: manual failover initial implementation.
Cluster: force AUTH ACK on manual failover.

When a slave requests masters vote for a manual failover, the
REQUEST_AUTH message is flagged in a special way in order to force the
masters to give the authorization even if the master is not marked as
failing.
Cluster: check that a MF is in progress in manualFailoverCheckTimeout().

Otherwise it is always detected as a manual failover timed out.