Hard failover gets interrupted by auto-failover

It so happens that we can interrupt a hard failover of one node to then auto-failover the same node. ns_server should not be overriding a hard failover request from the user to do an auto-failover.

Before 6.5 hard failover was a synchronous operation, that would be completed in the idle state.

For durability we made changes to hard failover which potentially could take a long time to complete. Hence, we perform this failover in rebalancing state and return immediately to the user.

This allows hard failover to be interrupted, by auto-failover, which is not desirable.