etcdserver: apply config change on cluster store

goal:
make changes on cluster store according to config change

non-goal:
check on adding exsiting node / removing non-existant node

for #1016 
The reason for this is the change of line 269. I think you(also me) want to encapsulate the `encoding`/`decoding` logic to lowest level. Also it makes sense to let clusterStore understand the fields of a member.