refactor command.go: commands do not rely on the etcdStore singleton. So we can seprate command into a package in the furture.
refactor command.go,server.go: add raftWrapper as context, totally get rid of reference in command.go
Merge pull request #212 from xiangli-cmu/newStore

refactor command.go: commands do not rely on the etcdStore singleton.
refactor remove raft singleton
Merge pull request #214 from xiangli-cmu/newStore

WIP refactor remove raft singleton