Don't clear entire state after moving to the failed or completed state

[mickel8]

When moving to the failed or completed state, we have been clearing the whole state, in particular we have been removing unused candidate pairs, local candidates and sockets.

Such approach makes it hard to debug connectivity issues, as when we moved to the failed state, we cannot inspect checklist pairs, requests sent and responses received (tracking requests and responses is not implemented yet).

After these changes, when the PC moves to the failed state, one will be able to get ICE transport stats and try to analyze what went wrong.

[codecov]

Codecov Report

Attention: Patch coverage is 81.41026% with 29 lines in your changes missing coverage. Please review.

Project coverage is 83.21%. Comparing base (125a04c) to head (2b1b4a5).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/ex_ice/priv/ice_agent.ex	76.04%	23 Missing ⚠️
lib/ex_ice/priv/conn_check_handler/controlled.ex	86.36%	3 Missing ⚠️
lib/ex_ice/priv/conn_check_handler/controlling.ex	81.25%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #69      +/-   ##
==========================================
+ Coverage   83.12%   83.21%   +0.09%     
==========================================
  Files          25       25              
  Lines        1564     1638      +74     
==========================================
+ Hits         1300     1363      +63     
- Misses        264      275      +11     

Files with missing lines	Coverage Δ
lib/ex_ice/priv/candidate_base.ex	93.33% <ø> (ø)
lib/ex_ice/priv/checklist.ex	100.00% <100.00%> (ø)
test/support/transport/mock.ex	80.00% <100.00%> (+1.42%)	⬆️
lib/ex_ice/priv/conn_check_handler/controlled.ex	91.78% <86.36%> (+15.91%)	⬆️
lib/ex_ice/priv/conn_check_handler/controlling.ex	88.88% <81.25%> (-7.27%)	⬇️
lib/ex_ice/priv/ice_agent.ex	85.57% <76.04%> (-1.00%)	⬇️

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