move response callbacks to a tween

[mmerickel]

• Move response callbacks to a tween pyramid.tweens.response_callbacks.
• Adjust config.add_tween to, by default, always add tweens with under='response callbacks', over=EXCVIEW.
• Document issues when adding tweens under EXCVIEW where cyclic dependencies are possible unless over='MAIN' is defined. This is similar to how config.add_view_deriver works with its defaults.
• Add changelog about bw-incompatibility where if a user is doing add_tween(under=EXCVIEW) then they should update their code to do add_tween(under=EXCVIEW, over=MAIN).

This work will help the TM wrap the response callbacks in #2606.

[mmerickel]

@mcdonc what is the status of this after pycon sprints?

[mmerickel]

@mcdonc is there any partial work on this from pycon that you could push? Otherwise I'm going to start working on this from scratch, but would much prefer to continue anything you may have already begun.

[mmerickel]

Okay, I'm going to start working on this myself.

[mmerickel]

#1336 covers some rants about why this may be a bad idea. At the end of the day it boils down to the invoke_subrequest API which is a huge problem because it breaks a lot of typical expectations of how a request is processed. I think that API needs to be reworked before we can work on this.

[digitalresistor]

@mmerickel I believe this may be closed based upon comments here:

Pylons/pyramid_tm#58
#2762