Read from cache again in ObservableQuery#getCurrentResult.

[benjamn]

This partially reverts two commits from PR #5565, preserving related improvements that have happened in the meantime:

• Revert "Remove partial field from ApolloCurrentQueryResult type."
  This reverts commit ca2cef6.
• Revert "Stop reading from cache in ObservableQuery#getCurrentResult."
  This reverts commit 01376a3.

One of the motivations behind the original changes was to lay the foundations for eventually supporting asynchronous (specifically, Promise-based) cache reads, but that form of asynchrony is no longer a goal of Apollo Client 3.0 or any planned future version (there will be other ways to deliver results asynchronously, without requiring every cache read to return a Promise), so there's not as much benefit to weakening getCurrentResult in this way.

On the other hand, allowing getCurrentResult to read synchronously from the cache ensures that custom field read functions can return new results immediately, which helps with issues like #5782.

Immediate cache reads have also been the behavior of Apollo Client for much longer than #5565, which makes this change appealing from an ease-of-upgrading perspective.

[hwillson]

Looks great @benjamn - thanks!

[hwillson]

@benjamn I re-ran the tests to get them to pass; we have a timing issue buried in one of our polling tests, which I've made a note of and will address separately.

[jkonowitch]

I was just about to file a bug report. This PR fixes the issue, but wanted to document it here in case you had been unaware of this specific problem.

Using values in the cache as default arguments to queries (via the @client @export(as: "whatever") directives), did not work.

https://github.com/jkonowitch/react-apollo-error-template/tree/export-error

I set up a repro with the error template. I slighly extended the "server" schema so that the people query takes an argument - orderBy.

I then added activeSortOrder as a client side schema extension.

The following query did not return a result, whereas it should return the people entities, sorted correctly.

const ALL_PEOPLE = gql`
  query AllPeople($orderBy: SortOrder!) {
    activeSortOrder @client @export(as: "orderBy")
    people(orderBy: $orderBy) {
      id
      name
    }
  }
`;

This works as expected with beta.23 👏