Fix EPERM and socket mismatch between selected and succeeded pairs when using bridge (Docker) interfaces.

[Karolk99]

Closes #77 #83

[Karolk99]

The problem with the test test/integration/p2p_test.exs:10 also occurs on the master branch. After initial debugging, the problem occurs when:

1. Ice state changes to :connected
2. p2p test tries to send a message using ICEAgent.send_data
3. Before handling send_data ICEAgent tries to nominate the only existing candidate pair (pair state changes to :waiting than :in_progress
4. ICEAgent handles send_data message and crashes because there is no pair with state :succeeded

[Karolk99]

Potential fixes:

• only check if valid? attribute is true
• Don't change pair state to :waiting/:in_progress when trying to nominate a pair.

[mickel8]

Normally, I would give an approve but this time we MUST be very verbose in comments both in the main logic as well as in tests. We spent a lot of time trying to find out what's going on with eperm and how to solve it so it MUST be very well described in comments for future reference. Especially, if it turns out that we are wrong

[mickel8]

The problem with the test test/integration/p2p_test.exs:10 also occurs on the master branch. After initial debugging, the problem occurs when:

Nice catch!! Let's move this to a separate PR

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.91%. Comparing base (7552d06) to head (be6980c).
Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #84      +/-   ##
==========================================
+ Coverage   83.88%   85.91%   +2.02%     
==========================================
  Files          25       25              
  Lines        1837     1874      +37     
==========================================
+ Hits         1541     1610      +69     
+ Misses        296      264      -32     

Files with missing lines	Coverage Δ
lib/ex_ice/priv/ice_agent.ex	87.77% <100.00%> (+1.69%)	⬆️

... and 3 files with indirect coverage changes

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