Make app.close() async, unify 'server' interface

[jlaine]

The "_create_server" method returned something very different depending on whether the transport was UDP or TCP/WS. In the UDP case what was returned was in fact the protocol, which has neither "close" nor "await_closed()" methods.

These kind of differences should not be exposed outside the API boundary, as they mean special cases in our users' code.

Also, make app.close() the preferred way of shutting down the application.

[codecov-io]

Codecov Report

Merging #57 into master will increase coverage by 0.04%. The diff coverage is 30%.

@@            Coverage Diff             @@
##           master      #57      +/-   ##
==========================================
+ Coverage   27.51%   27.56%   +0.04%     
==========================================
  Files          15       15              
  Lines        1443     1455      +12     
  Branches      259      260       +1     
==========================================
+ Hits          397      401       +4     
- Misses       1033     1041       +8     
  Partials       13       13

Impacted Files	Coverage Δ
aiosip/protocol.py	20.79% <0%> (-0.21%)	:arrow_down:
aiosip/peers.py	20.8% <33.33%> (+0.65%)	:arrow_up:
aiosip/application.py	35.22% <50%> (ø)	:arrow_up:

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[jlaine]

Any chance of merging this? Thanks!

[jlaine]

By the way I'm having a hard time understanding how the test suite is supposed to be run, the vast majority of checks get marked as "skipped"