Deal with missing data

[lukel97]

Imagine 2 queries like:

query a {
   foo {
       __typename id
       baz { qux }
   }
}
query b {
   foo {
       __typename id
       baz { qan }
   }
}

where baz is a list of objects without any ID.
Query A is executed and the cache is stored as:

   foo:1 => {
       baz => [{ qux => 42 }]
   }

And now query B is executed, updating the cache to:

   foo:1 => {
       baz => [{ qan => 42 }]
   }

So we go to update query A's response with the new data in the cache, but qux isn't present on baz!
This is called the missing data problem apparently, and there are problems with it in both apollo and relay:

facebook/relay#2237 (comment)
apollographql/apollo-client#6375

https://github.com/apollographql/apollo-client/blob/4e147b579bff210dbbc884d918edb1e69c2cfa9f/src/cache/inmemory/readFromStore.ts#L261-L277

Consistency and refetching

In this specific example however, it looks like apollo refetches the query. Should investigate as to how/why it does that