Fix EndTranscodingSession() call and potential race

[cyberj0g]

Addresses #2732

Despite all efforts, I was yet unable to reproduce the error locally, however, I added a fix for related issue, and the fix which potentially should address this issue.

Findings

• EndTranscodingSession() call here was failing, because of context cancelled error. It's not clear why this context, which is, presumably, goes all the way down from HandlePush(), is causing that error on gRPC communication, but it's consistently reproducible. Fixed by changing it to Context.Background().
• EndTranscodingSession() is called again here on stream end. There might be a race condition between receiving from the stop channel and locking the mutex to cleanup the session on T. We had reports of noop transcoding jobs (empty segments?), could it be that it caused EndTranscodingSession() to be called twice in a very quick succession, if above context cancelled issue doesn't happen on all systems? Addressed by removing the session id from the map right in EndTranscodingSession() with mutex locked.

[codecov]

Codecov Report

Merging #2735 (def4696) into master (af495ed) will increase coverage by 0.01272%.
The diff coverage is 50.00000%.

@@                 Coverage Diff                 @@
##              master       #2735         +/-   ##
===================================================
+ Coverage   56.33553%   56.34825%   +0.01272%     
===================================================
  Files             88          88                 
  Lines          19146       19147          +1     
===================================================
+ Hits           10786       10789          +3     
+ Misses          7768        7767          -1     
+ Partials         592         591          -1     

Impacted Files	Coverage Δ
core/orchestrator.go	78.20690% <0.00000%> (+0.13793%)	⬆️
core/lb.go	80.11696% <50.00000%> (+0.70520%)	⬆️
server/rpc.go	72.02381% <100.00000%> (ø)

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Impacted Files	Coverage Δ
core/orchestrator.go	78.20690% <0.00000%> (+0.13793%)	⬆️
core/lb.go	80.11696% <50.00000%> (+0.70520%)	⬆️
server/rpc.go	72.02381% <100.00000%> (ø)