etcdserver: refactor cluster and clusterStore

Integrate clusterStore into cluster, and let cluster become the source of
cluster info.

fixes #1354 

early feedback welcome
@jonboulle i don't know what is the better way.. NewClusterFromString is called twice in main and etcdserver pkg, so i want to keep it. NewClusterFromMemberInfos uses data with better format, or I have to convert it into string, and parse it out again.. Any thoughts?