Fix race condition in XHR and handle other abort/open scenarios

This fixes issue #3630
A short summary of the changes:
* Use generation id to cancel inflight requests
* Handles nested calls to abort, open, send inside handlers
* Adds XHRReleaseMsg to delay freeing XHR object till all
  inflight events are processed
* Change the ErroredMsg enum to be more symmetric with the returned
  Error enum
Update expectations for wpt tests under /XMLHttpRequest

Following tests are re-enabled:
* /XMLHttpRequest/send-network-error-async-events.sub.htm
* /XMLHttpRequest/send-network-error-sync-events.sub.htm
* /XMLHttpRequest/xmlhttprequest-timeout-worker-aborted.html
* /XMLHttpRequest/xmlhttprequest-timeout-worker-overrides.html
* /XMLHttpRequest/xmlhttprequest-timeout-worker-simple.html
* /XMLHttpRequest/xmlhttprequest-timeout-worker-twice.html

Following tests are disabled:
* /XMLHttpRequest/xmlhttprequest-timeout-worker-overridesexpires.html

The following tests are expected to timeout since window.postMessage is
not implemented yet:

* /XMLHttpRequest/xmlhttprequest-timeout-abortedonmain.html
* /XMLHttpRequest/xmlhttprequest-timeout-overrides.html
* /XMLHttpRequest/xmlhttprequest-timeout-overridesexpires.html
auto merge of #3732 : mukilan/servo/xhr-issue-3630, r=jdm

This fixes issue #3630
A short summary of the changes:
* Use atomic generation id to cancel inflight requests
* Handles nested calls to abort, open, send inside handlers
* Adds XHRReleaseMsg to delay freeing XHR object till all
  inflight events are processed
* Handles both timeout, errors and abort/open in a symmetric fashion
  i.e All inflight events will be cancelled for timeouts, aborts,
    errors and on calling open.
* Change the ErroredMsg enum to be more symmetric with the returned
  Error enum

I noticed a few possible changes that could make the code for fetch task simpler:
* We can remove the additional timer task and let the fetch task manage 
  its own timer (or maybe the resource loader can do this.)
* The CORS related steps could also be moved into the resource loader.
* Right now upload events are not support. This requires some support 
  from resource loader.