save failed mpeg7 signature and segment data for fast verification

[oscar-davids]

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Checklist:

• [X] Read the contribution guide
• [X] make runs successfully
• [ ] All tests in ./test.sh pass
• [ ] README and other documentation updated
• [ ] Pending changelog updated

[codecov[bot]]

Codecov Report

Merging #2451 (959e580) into master (d360781) will decrease coverage by 0.27796%. The diff coverage is 20.15209%.

:exclamation: Current head 959e580 differs from pull request most recent head c3d8207. Consider uploading reports for the commit c3d8207 to get more accurate results

@@                 Coverage Diff                 @@
##              master       #2451         +/-   ##
===================================================
- Coverage   55.02775%   54.74979%   -0.27797%     
===================================================
  Files             94          95          +1     
  Lines          19641       19843        +202     
===================================================
+ Hits           10808       10864         +56     
- Misses          8236        8375        +139     
- Partials         597         604          +7     

Impacted Files	Coverage Δ
cmd/livepeer/starter/starter.go	2.30769% <0.00000%> (-0.06856%)	:arrow_down:
cmd/livepeer_cli/livepeer_cli.go	0.00000% <0.00000%> (ø)
cmd/livepeer_cli/wizard_transcoder.go	0.00000% <0.00000%> (ø)
core/livepeernode.go	71.05263% <0.00000%> (-10.76556%)	:arrow_down:
drivers/gssave.go	0.00000% <0.00000%> (ø)
pm/stub.go	56.98113% <0.00000%> (-0.43333%)	:arrow_down:
server/ot_rpc.go	35.75581% <19.23077%> (+9.89910%)	:arrow_up:
server/broadcast.go	77.32975% <33.33333%> (-0.58880%)	:arrow_down:
pm/recipient.go	89.63964% <57.14286%> (-2.45338%)	:arrow_down:
server/handlers.go	53.51598% <73.91304%> (+0.43761%)	:arrow_up:
... and 7 more

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