replace waitUntil with async callback

[inexorabletash]

Inspired by the work over in https://github.com/inexorabletash/web-locks and suggested by @dominiccooney at TPAC 2016...

Rather than the confusing usage of waitUntil(), we could do something like:

db.transaction(scope, mode, async tx => {
  const store = tx.objectStore('s');
  const value = await store.get('key');
  const response = await fetch(`${url}?value=${value}');
  if (response.ok) {
    await store.put(await response.text(), 'stuff');
  }
  // tx commits when this async function's promise resolves
});

Lots of details to be worked out (e.g. what does transaction() return here? (IDBTransaction or Promise<void>) ? what do cursors look like, etc?)

(This is assuming we want to tackle promisifying transactions in the first place, vs. a completely revised API. Tracking it here for posterity.)

[jakearchibald]

I really like this as a way of 'fixing' the IDB API without breaking compatibility.

I think the return of transaction() can remain the same, but add a .complete promise to the IDBTransaction. The inner tx (shall we call it IDBPromiseTransaction?) won't have .complete, as it'd likely lead to deadlock.

I think we'd need something similar for opening the database:

const openRequest = indexedDB.open(name, version, async upgradeTransaction => {
  const store = upgradeTransaction.createObjectStore('blah');
  // etc
});

Again, IDBOpenDBRequest needs some kind of promise to get the IDBDatabase.