Move dialplan logic fully into Application, remove dialog router

[vodik]

Putting this up for feedback. I know you guys have been kinda busy, so take your time, still a few things left to do:

The PR removes the dialog router. I plan to move everything to using an iterator based API.

In order to do this, I created a new Request class to represent a new potential dialog. This now fed to the callbacks registered in the dialplan, and give them room to either reject or accept traffic. Should it be accepted, a Dialog is returned.

This returned dialog object should now behave exactly like the client side dialog, meaning future messages on that dialog should be read out by iterating (enabling re-REGISTER support, for example).

This breaks two features, authentication (can be retrofitted in, just haven't yet), and proxying.

Proxying will probably need a completely new implementation that's built around coroutines and iterators instead of routing hooks.

[codecov-io]

Codecov Report

Merging #74 into master will decrease coverage by 0.73%. The diff coverage is 80.35%.

@@            Coverage Diff             @@
##           master      #74      +/-   ##
==========================================
- Coverage   70.48%   69.74%   -0.74%     
==========================================
  Files          15       15              
  Lines        1484     1494      +10     
  Branches      263      262       -1     
==========================================
- Hits         1046     1042       -4     
- Misses        333      342       +9     
- Partials      105      110       +5

Impacted Files	Coverage Δ
aiosip/dialplan.py	60% <ø> (-26.67%)	:arrow_down:
aiosip/peers.py	68.29% <100%> (+1.27%)	:arrow_up:
aiosip/dialog.py	74.76% <100%> (-1.08%)	:arrow_down:
aiosip/transaction.py	61.71% <100%> (-0.36%)	:arrow_down:
aiosip/application.py	70.28% <79.24%> (+3.24%)	:arrow_up:
aiosip/utils.py	64.1% <0%> (-5.13%)	:arrow_down:
... and 1 more

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[ludovic-gasc]

It's OK for me.

[vodik]

Even though the test code got more complicated (for now), its also a net code reduction!

[vodik]

Merging so I can get the last big thing done - INVITE