Add close/1

[mickel8]

Add close/1 function.

This function irreversibly closes ICE agent but does not terminate its process. Later calls to getters (e.g. get_stats/1) are still allowed but the agent can no longer be used to establish a connection.

NOTE The best solution would be to modify ICE agent API to be similar to Erlang's :gen_udp API where, after closing a socket, :gen_udp functions return {:error, :closed} tuple. This however, would require a lot of changes. Other incorrect actions e.g. setting remote credentials in a wrong state, are also ignored i.e. no error is returned. If we find this problematic, we will modify ICE agent API in the future.

Related to: elixir-webrtc/ex_webrtc#218

[codecov]

Codecov Report

Attention: Patch coverage is 58.13953% with 18 lines in your changes missing coverage. Please review.

Project coverage is 84.75%. Comparing base (7552d06) to head (c13e5ad).

Files with missing lines	Patch %	Lines
lib/ex_ice/priv/ice_agent.ex	55.00%	18 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #81      +/-   ##
==========================================
+ Coverage   83.88%   84.75%   +0.87%     
==========================================
  Files          25       25              
  Lines        1837     1870      +33     
==========================================
+ Hits         1541     1585      +44     
+ Misses        296      285      -11     

Files with missing lines	Coverage Δ
lib/ex_ice/ice_agent.ex	65.75% <100.00%> (+4.32%)	⬆️
lib/ex_ice/priv/ice_agent.ex	85.92% <55.00%> (-0.16%)	⬇️

... and 2 files with indirect coverage changes

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