refactor watcher.go
refactor node.go; use once
fix node.go race between expire and update
comments node.go
refactor separate kvpair to kvpair.go; simplify sorting interface
refactor add node.ExpirationAndTTL
refactor store.go remove unwanted print
refactor store.go update
refactor store.go remove extra assignment
Merge remote-tracking branch 'xiangli-cmu/newStore' into 0.2