etcdserver: support GET on admin endpoint
etcdhttp: sort member
Merge pull request #1365 from xiangli-cmu/admin_get

etcdserver: support GET on admin endpoint
etcdserver: refactor cluster and clusterStore

Integrate clusterStore into cluster, and let cluster become the source of
cluster info.
etcdserver: MemberFromName -> MemberByName
etcdserver: Member.Pick -> Member.PickPeerURL
etcdserver: generate id when new cluster
etcdserver: use path.Join for member key in cluster
etcdserver: remove NewClusterFromMemberInfos
etcdserver: rebase on master