selection: Clear known sessions if none of them has good enough latency score

[leszko]

fix https://linear.app/livepeer/issue/ENG-2051/investigate-warnings-no-orchestrators-passed-max-price-filter

Problem: We never cleaned the known sessions, even if all of them had too low latency score to use; example of what sometimes happens:

1. Broadcaster accumulated 8 known sessions, but all had too low latency score
2. Session refresh was not triggered because we had a lot of known session in the pool
3. At the same time, we didn't have sessions in unknown session pool
4. Selection checked that it cannot use the known session and tried to select from unknown sessions (but the pool was empty or there were no sessions fulfilling the condition of max price or perf score)

[linear]

ENG-2051 Investigate warnings "No Orchestrators passed max price filter"

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 57.38346%. Comparing base (45652c4) to head (97bf818).
Report is 1 commits behind head on master.

Additional details and impacted files

@@                 Coverage Diff                 @@
##              master       #3086         +/-   ##
===================================================
+ Coverage   57.38077%   57.38346%   +0.00269%     
===================================================
  Files             92          92                 
  Lines          15852       15853          +1     
===================================================
+ Hits            9096        9097          +1     
  Misses          6147        6147                 
  Partials         609         609                 

Files with missing lines	Coverage Δ
server/selection.go	93.16239% <100.00000%> (+0.05894%)	⬆️

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Files with missing lines	Coverage Δ
server/selection.go	93.16239% <100.00000%> (+0.05894%)	⬆️

[victorges]

Makes sense to me on the code snippet that was changed, but FTR I'm not super familiar with that entire session management logic.

LGTM