Update the multiSet to update subscribers in a more optimized way

[yuwenmemon]

Problem

Currently, the way multiset works is that it notifies subscribers of any changes to the onyx keys in a loop:

react-native-onyx/lib/Onyx.js

Lines 985 to 992 in f96fa2d

	_.each(data, (val, key) => {
	// Update cache and optimistically inform subscribers on the next tick
	cache.set(key, val);
	notifySubscribersOnNextTick(key, val);
	});
	return Storage.multiSet(keyValuePairs)
	.catch(error => evictStorageAndRetry(error, multiSet, data));

The performance of this can be improved in relation to collections, where we can see if all the keys are part of a collection, and if so - use keysChanged to update subscribers in a more optimized way.

Solution

Either:

1. Update multiset to support the above, or
2. Create a setCollection that is optimized for collections

cc @marcaaron