[peers] make Peer.close() async, actually use connection_lost

[jlaine]

The previous closing mechanism:

• did not take into account that transports invoke connect_lost asynchronously
• introduced an unfortunate coupling between Peer and Connector, where the peer needed to know about it's connector

The new mechanism is clearer as we clearly distinguish "we want to close the transport" and "the transport has closed". It also allows for clients and servers to shutdown cleanly regardless of each peer's transport.

We also stop pretending a Peer can be used as a context manager.

[codecov-io]

Codecov Report

Merging #61 into master will increase coverage by 0.22%. The diff coverage is 96.87%.

@@            Coverage Diff             @@
##           master      #61      +/-   ##
==========================================
+ Coverage   71.22%   71.44%   +0.22%     
==========================================
  Files          15       15              
  Lines        1449     1443       -6     
  Branches      259      260       +1     
==========================================
- Hits         1032     1031       -1     
+ Misses        309      307       -2     
+ Partials      108      105       -3

Impacted Files	Coverage Δ
aiosip/peers.py	69.4% <100%> (+1.15%)	:arrow_up:
aiosip/protocol.py	62.37% <80%> (ø)	:arrow_up:

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[ovv]

I need to take some time to go back into the code, it's been a few month since I last played with it. I'll try to do that next week. Hopefully then things will be clearer

[ovv]

@vodik I haven't had time to look into it yet but if you want feel free to merge this.

[vodik]

introduced an unfortunate coupling between Peer and Connector, where the peer needed to know about it's connector

I'm pretty confident we can overcome this with some more refactoring, which will have to happen anyways since we should support UDP+TCP.

So nice work. Merging.