Fixing the cache update logic

[cihadturhan]

Hello,
First of all, I've used this library extensively for last week and I want to thank for your hard work. It works great!

Secondly, I've used apollo client with react, react native and vue so I'd like to ask about cache update logic OR report a bug if it's a design flow. Here is an example:

• Page A
  -- Query (messages)

• Page B
  -- Mutation (addMessage)
  --- Use update() callback in Mutation widget. Update "messages" cache and run cache.save()

This scenario works fine 👍 Let's skip to second scenario

• Page A
  -- Query (messages)

• Page B
  -- receive an event (e.g. push notification with data)
  -- Update "messages" cache and run cache.save()

In this case, I would expect the Query in "Page A" to be updated but it's not. 👎 I've spent almost two days to understand what's wrong.

After digging code, it turns out there is also a local snapshot for every Query widget.
If I change the cache externally, a local snapshot from AsyncSnapshot will not be updated.
I realized snapshot is only updated with Mutation callbacks: update, onError and onCompleted

If I want to update a Query snapshot externally, I manually force cache update with
client.queryManager.rebroadcastQueries()
This was my aha moment.

Is this a design choice? I believe, rebroadcastQueries should run after ever cache.write().
Correct?

[micimize]

Consolidating into #728