etcdserver: Calculate IDs for nodes solely on PeerURLs

Removes the notion of name being anything more than advisory or
command-line grouping, and adds checks for bootstrapping the command
line. IDs are consistent if the URLs are consistent.
I do not think we need to keep a local copy of member. We should always get a copy from the clusterStore. 
Can we change the Name -> ID instead of Name -> LocalMember?