[bug] Strong consistency cannot be guaranteed after a two-node cluster adds a follower

[TCeason]

Now rpl_semi_sync_master_timeout just settings in MySQL config and leader of two-node cluster.

If we just add a new follower to a two-node cluster but do not restart MySQL, now the value of does not change to a large number.

So semi-sync maybe degrades to async.

Detailed description:

If the role of the new node that we will add to the two-node cluster is FOLLOWER, we need to modify rpl_semi_sync_master_timeout.

But If It is IDLE, we should not modify rpl_semi_sync_master_timeout.