A new and improved IResource

[lvh]

There are a number of problems with IResource. A (possibly non-exhaustive) list:

1. #3711 (returning deferreds from render methods)
2. #3621 (returning deferreds from getChild and friends)
3. #288 (accessing uploaded data while in progress)

None of these problems can be fixed without breaking IResource, which is something we don't want to do. As a result, we need a new IResource interface.

The goal of this ticket is to produce that interface, and to introduce fixes for each individual ticket into an integration branch related to this interface. That way, once all the tickets have been addressed, we can just merge this into trunk.