[Epic] At normalization time, execute code

[rbalicki2]

There are several reasons we may choose to execute code when a field is normalized:

• Some fields will start subscriptions as soon as possible, which is to say, at normalization time.
• Fake deferred fields are executed as soon as possible (normalization time)
• You may want to keep some external state up-to-date

We should add this ability.

This may subsume: at normalization time, execute resolvers and write their result to the store. Should that be done lazily?

[rbalicki2]

Duplicate of #78