Add ability to abourt outstanding requests of a Resource

Resource now have an `abort()` function.

I philosophized a bit about problems we had when moving from chunk to chunk in #237 and #242, but I wasn't quite right there. Outstanding requests, that trigger obsolete callbacks where the problem.

`morph` can now abort outstanding calls when we enter a new chunk - this seems to solve all the problems we had.

Fixes #237 
Fixes #242