V2 - allow for async modifyUp()s

[dxinteractive]

Will be great for async validation. May require #238 because chances are that we wouldn't want to merely delay a result, but also set parcel data upon fetch and error.

[dxinteractive]

This will require the reducer to be much smarter.

• Do V2 - simplify modify methods #284 first
• modify methods can return an effect function that passes an update function. This can be called as many times as required, however long after the effect function is called
• effect needs to only be called once for each modifier parcel, after the reducer has run
  • Might need to dedupe if the same modifier parcel effect is being run at different depths. Best approach would be to debounce the effect function as it's passed in
• reducer will need to be passed a callback that it calls with the result, like an observable, so that it can push multiple changes out
  • Would need to look up the latest version of the parcel in the registry and probably just dispatch the change request on it
  • Add option to memoize the effect
• usages of the reducer will need to match that same inversion of control too
  • wouldn't need to do this if subsequent calls go via a dispatch on the parcel
• firing off the effect can be done by taking the action stack at the point where the effect is, cutting off the leaf-ward actions and firing an update action instead.