fix deadlock when GetChannelMediumOptions is used

[FZambia]

When GetChannelMediumOptions is used addSubscription call acquired subLock mutex, HandlePublication tried to acquire the same mutex concurrently before addSubscription call released one. This resulted into deadlock.

Also, fixing behaviour of ping_pong_duration_seconds to not count clients which did not sent pong in time as this resulted in negative values.

[codecov]

Codecov Report

Attention: Patch coverage is 28.00000% with 18 lines in your changes missing coverage. Please review.

Project coverage is 81.25%. Comparing base (70fd357) to head (71cf2c3).

Files	Patch %	Lines
node.go	18.18%	17 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #404      +/-   ##
==========================================
- Coverage   81.46%   81.25%   -0.21%     
==========================================
  Files          40       40              
  Lines        7174     7193      +19     
==========================================
+ Hits         5844     5845       +1     
- Misses        952      972      +20     
+ Partials      378      376       -2     

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