deleteDatabase's "versionchange" should be deferred if upgrade is running

[inexorabletash]

The "steps for opening a database" ensure that if an upgrade transaction is running that a subsequent open request with a higher version is "deferred" until the previous request is completed. And it should not be necessary to assign a "versionchange" handler in your "upgradeneeded" handler.

For example, the following should work:

indexedDB.open('db', 1).onsuccess = e => {
  let db = e.target.result;
  db.onversionchange = e => db.close(); // Get out of the way.
};
indexedDB.open('db', 2).onblocked = e => alert('should not happen!');

It's reasonable to expect the same thing for deletes:

indexedDB.open('db', 1).onsuccess = e => {
  let db = e.target.result;
  db.onversionchange = e => db.close(); // Get out of the way.
};
indexedDB.deleteDatabase('db').onblocked = e => alert('should not happen!');

... but the spec does not require that. Firefox behaves as expected. Chrome does not but we're fixing to match Firefox.

We should add a clause to the "steps for deleting a database" to wait for running upgrades.

[inexorabletash]

Should be resolved by d1147cb - take a peek?