etcdserver: etcdserver generates the ID when adding new member.

When adding new member, the etcdserver generates the ID based on the current time
and the given peerurls. We include time to add the uniqueness, since the node with
same peerurls should be able to (add, then remove) several times.
@unihorn Do you think we can change the AddMember API? we only need to care about raft Attributes + ID actually.