[WIP] Add @defer support

[clarencenpy]

Changes:

• Expose loadingState on the Query component for deferred queries.
• When a deferred query component is unmounted, do not tear down the subscription so deferred fields can continue to stream in.

[clarencenpy]

This build is failing as it depends on changes in apollo-client #3686. Can we release an alpha version of AC, and pin the dependency here?

[hwillson]

Looks good @clarencenpy! Yes, we can definitely get an alpha of AC out. It looks like we'll need an Apollo Link alpha out first though, so we'll revisit this after the related @defer Apollo Link and Apollo Client PR's are ready to go. Thanks!

[joeflack4]

+1

[hwillson]

We're going to close this PR for now, and re-open it when we're ready to dive back into the @defer work (when the necessary Apollo Server request pipeline changes have been made, to better support @defer functionality).