Expose client-only level fields in new APIs

[wincent]

This issue replaces #114, which was the initial RFC issue for how to deal with client state in Relay. Since then, we have undertaken a big rewrite, leading to a new core that has low-level support for client-only fields. The purpose of this issue, then, is to track the exposure of these low-level features in the higher-level API. At the time of writing, some pieces of the new core and API have started making their way onto the master branch, with more to come in the following weeks/months.

[wincent]

Closing this as too old, stale, vague and high-level. @wincent, feel free to re-open with more detail if this is still important to you.