Clean up inactive sessions

[leszko]

Not cleaning up the sessions causes quite a big memory leak which in effect causes OOMKilled

It's an alternative approach to #3163 in which, instead of keeping the timestamp of the last usage, we add cleanup into the flow.

Note that I'm 100% we won't have some corner cases, like segment was returned with a delay and now we cannot find the session. Saying that, it should not crash anything. Also, I did quite a lot of local dev chain tests, so I think it should work fine.

My plan is to get it merged and then deploy to one region for some time to see if we see any regression.

[codecov]

Codecov Report

Attention: Patch coverage is 62.50000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 57.38077%. Comparing base (eec6ed3) to head (e5d01b2).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
pm/sender.go	0.00000%	2 Missing ⚠️
pm/stub.go	0.00000%	2 Missing ⚠️
server/broadcast.go	83.33333%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##              master       #3166         +/-   ##
===================================================
- Coverage   57.38905%   57.38077%   -0.00828%     
===================================================
  Files             92          92                 
  Lines          15841       15852         +11     
===================================================
+ Hits            9091        9096          +5     
- Misses          6142        6147          +5     
- Partials         608         609          +1     

Files with missing lines	Coverage Δ
server/rpc.go	70.81712% <ø> (ø)
pm/sender.go	97.87234% <0.00000%> (-2.12766%)	⬇️
pm/stub.go	56.89655% <0.00000%> (-0.49475%)	⬇️
server/broadcast.go	79.97870% <83.33333%> (-0.06422%)	⬇️

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Files with missing lines	Coverage Δ
server/rpc.go	70.81712% <ø> (ø)
pm/sender.go	97.87234% <0.00000%> (-2.12766%)	⬇️
pm/stub.go	56.89655% <0.00000%> (-0.49475%)	⬇️
server/broadcast.go	79.97870% <83.33333%> (-0.06422%)	⬇️

[mjh1]

Simple enough change at least 👍 but yep sounds good on monitoring one region first