Backport of "Accumulated SRT fixes" into 1.20

Backport of !3156 (merged) "Accumulated SRT fixes".


  • b6974b6a srt: Replace stats accumulation with naive byte counting
  • a3cc5cf2 srt: Simplify socket stats
  • 4e05100e srt: Clean up error handling
  • 3c23c16f srt: Clean up poll/sock lifecycle
  • d575a411 srt: Use simpler list operations for callers
  • 424b331a srt: Remove callers for which srt_bstats fails

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