chore(watch/cancel): protect watcherHub with a lock
chore(watch/cancel): rename watcher.Cancel() to watcher.Remove()
Merge branch 'cancel' into stream_cancel

* cancel:
  chore(watch/cancel): rename watcher.Cancel() to watcher.Remove()
  chore(watch/cancel): protect watcherHub with a lock

Conflicts:
	store/watcher_hub.go
	store/watcher_test.go