Make debug segment count configurable

[mjh1]

I've been finding it necessary to save more than the first 10 segments to be able to debug the different input modes we have in the app. Make it configurable so we can easily adjust it temporarily.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 31.65527%. Comparing base (1b6298d) to head (f1cf907).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
server/mediaserver.go	0.00000%	2 Missing and 1 partial ⚠️
server/ai_live_video.go	0.00000%	2 Missing ⚠️
cmd/livepeer/starter/flags.go	0.00000%	1 Missing ⚠️
cmd/livepeer/starter/starter.go	0.00000%	1 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##              master       #3714         +/-   ##
===================================================
- Coverage   31.67174%   31.65527%   -0.01647%     
===================================================
  Files            158         158                 
  Lines          47645       47654          +9     
===================================================
- Hits           15090       15085          -5     
- Misses         31659       31671         +12     
- Partials         896         898          +2     

Files with missing lines	Coverage Δ
core/livepeernode.go	60.22099% <ø> (ø)
cmd/livepeer/starter/flags.go	0.00000% <0.00000%> (ø)
cmd/livepeer/starter/starter.go	7.19424% <0.00000%> (-0.02163%)	⬇️
server/ai_live_video.go	0.00000% <0.00000%> (ø)
server/mediaserver.go	67.35004% <0.00000%> (-0.15377%)	⬇️

... and 2 files with indirect coverage changes

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Files with missing lines	Coverage Δ
core/livepeernode.go	60.22099% <ø> (ø)
cmd/livepeer/starter/flags.go	0.00000% <0.00000%> (ø)
cmd/livepeer/starter/starter.go	7.19424% <0.00000%> (-0.02163%)	⬇️
server/ai_live_video.go	0.00000% <0.00000%> (ø)
server/mediaserver.go	67.35004% <0.00000%> (-0.15377%)	⬇️

... and 2 files with indirect coverage changes

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