`Onyx.update()` should return a `Promise`

[marcaaron]

Most other Onyx methods return a Promise so this feels like an implementation oversight. However, there's also a practical advantage in that you might want to apply some updates in an atomic way and it's not possible due to the way mergeCollection() is implemented.

e.g.

Onyx.update([
    {onyxMethod: 'merge', value: {isLoading: false}, key: 'someKey'},
    {onyxMethod: 'mergecollection', value: {test_1: {a: 'a'}}, key: 'test_'},
]);

Although called at the same time, this code will notify the subscriber of someKey first and we might assume the data from mergecollection is available as well - but there are no guarantees about this (and it is most likely not available).

If Onyx.update() returns a promise we can guarantee the correct order of events by doing something like:

Onyx.update(dataUpdate).then(() => Onyx.update(successUpdate));

This is much more predictable and will help solve the bug described here: Expensify/App#11726 (comment)

An future improvement could also be to make these updates atomic somehow - but I'm less sure what the solution for this would look like given the current state of Onyx's API.