etcdserver: refactor cluster and clusterStore

Integrate clusterStore into cluster, and let cluster become the source of
cluster info.

fixes #1354 

early feedback welcome
Offtopic: We ignore the initial-cluster-name in the discovery case? Then probably we shall change the initial-cluster-name to initial-cluster-token. We never print out the cluster-name as far as i can see. and do we want it to be descriptive in the future? If so, we probably should not use discovery url as the cluster-name in discovery case.

/cc @barakmich @philips @jonboulle @unihorn