Don't terminate PeerConnection after moving to the failed state

[mickel8]

We have been terminating PeerConnection after it moved to the failed state. While this approach was easy to implement, it had two drawbacks:

• once PeerConnection failed, we couldn't gather stats via get_stats as the process was no longer alive
• we couldn't perform ICE restart

This PR keeps peer connection alive even when it moves to the failed state. This is pretty big breaking change as a lot of our examples and demo apps relies on linking to the PeerConnection process and waiting until it crashes.

Things to note:

• when PC moves to the failed state (no matter this is because of DTLS or ICE transport fails), most of the operations are still allowed. E.g. user can still successfully call addTrack, addTransceiver or createOffer even though the connection might not be able to restart (that's the case when DTLS fails, or at least we don't support DTLS restart).
• sending RTP, RTCP, or data via DataChannel in state failed is silently discarded - silently because there is a RC between calling send_rtp and others, and moving to the failed state so such a log could appear pretty often
• closing web browser's peer connection no longer terminates our peer connection process. So far, closing web browser's peer connection was causing DTLS alert (probably close_notify) that was resulting in the termination of DTLS transport process. From now, we just move DTLS transport to the failed state

Relevant specs:

• W3C WebRTC sec. 5.5 - https://www.w3.org/TR/webrtc/#rtcdtlstransport-interface

[codecov]

Codecov Report

Attention: Patch coverage is 44.44444% with 5 lines in your changes missing coverage. Please review.

Project coverage is 88.51%. Comparing base (241931e) to head (fff797c).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/ex_webrtc/peer_connection.ex	25.00%	3 Missing ⚠️
lib/ex_webrtc/dtls_transport.ex	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #202      +/-   ##
==========================================
+ Coverage   88.38%   88.51%   +0.12%     
==========================================
  Files          49       49              
  Lines        2531     2533       +2     
==========================================
+ Hits         2237     2242       +5     
+ Misses        294      291       -3     

Files with missing lines	Coverage Δ
lib/ex_webrtc/dtls_transport.ex	84.17% <60.00%> (-1.23%)	⬇️
lib/ex_webrtc/peer_connection.ex	86.40% <25.00%> (+0.14%)	⬆️

... and 2 files with indirect coverage changes

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[mickel8]

This one should be fired before connection state change