Allow DeferredContext.addActionFinish to record the result.

[tomprince]

Perhaps by specifying the key to log to?

This is particularly useful, since there isn't an easy way to call a function in a callback, without disturbing the response. (https://twistedmatrix.com/trac/ticket/6292)

[itamarst]

Hm. Not sure addActionFinish is only place you might want to add results. Might want to add them inside callbacks too...

[itamarst]

Or at a minimum in between callbacks?

d = DeferredContext(...)
d.addCallback(f)
d.extractActionResults(lambda value: {"field": value.field})
d.addActionFinish()