[recoil-sync] Support transactional updateItems() in RecoilSync

[luishdz1010]

When syncing from an external store to recoil in the listen prop of RecoilSync, each call to updateItem(k, v) notifies components associated with each atom's key. However, since not all the state is synced yet (more synchronous updateItem() calls pending), some components will observe a "partially" synced state, which can cause errors.

In my case, this is making some components observe deleted state that would otherwise be consistent if all updates were applied before components get notified.

Using updateAllKnownItems might work, but in my case, the number of atoms is vast and updates are often, so this will do a lot of unnecessary work (assuming updateAllKnownItems has transaction semantics).

[drarmstr]

That should be straight-forward if someone want to submit a PR? Both updateItem() and updateAllKnownItems() are implemented in terms of updateItems() which just isn't exposed yet.