feat add ttl heap
feat add update heap test
refactor add newTTLKeyHeap function
refactor use time.IsZero
fix iszero
refactor add push/pop function
refactor use push
fix save and recovery
feat add heap top
init sync to delete expiring keys
refactor clean up
fix bug in update ttl
feat wathch for expiring need to be pending
fix watcher_hub
fix cleanup
fix sinceIndex
feat upgrade get_handler
refactor add comments for receiving expire commands
refactor comments on IsPermanent
refactor move sync command into etcd