When promoting a learner member we should not count already a voting
member, but take only into account the number of existing voting
members and their current status (started, unstarted) when taking the
decision whether a learner member can be promoted.
Before this change, it was impossible to grow from a quorum N to a N+1
through promoting a learning member.
Fixes: #11633
In case of URLs that are synonyms, the current lexicographic sorting
and compare of the URLs fails with frustrating errors. Make sure to do
a full comparison between every set of PeerURLs before failing.
Fixes#11013
1. Maintenance API MoveLeader() returns ErrBadLeaderTransferee if
transferee does not exist or is raft learner.
2. etcdserver TransferLeadership() only choose voting member as
transferee.
Added IsLearner field to etcdserver internal Member type. Routed
learner MemberAdd request from server API to raft. Apply learner
MemberAdd result to server after the request is passed through Raft.
