Log performance.parsoid.* events broadcast by VE

* 'VisualEditorDOMRetrieved' is now 'performance.parsoid.domLoad' (I54602394e).
* Use ve.trackSubscribe now that I3b58ce0f4 has been merged, but add safety
  check to make it easier and safer to deploy.
* Lazy-load schemas (i.e., only when a suitable event is ready to be logged).
* Log performance.parsoid.domSave events.

Change-Id: I8c2a25c761c2c7ee314e1e255af1fc264d2b2238