Bug-fixes

* Fixed race conditions posting Tap events (bucket needs to be locked
  while iterating its .tapFeeds)
* Fixed closing a TapFeed (old implementation totally didn't work)
* Keep JSMapFunctions from leaking (add a finalizer to a view to stop
  the JSMapFunction's goroutine)
* Keep Buckets from leaking (when closed, a bucket should remove itself
  from the global `buckets` map)