save failed mpeg7 signature and segment data for fast verification

[oscar-davids]

What does this pull request do? Explain your changes. (required)

Related to this issue

Specific updates (required)

How did you test each of these updates (required)

Does this pull request close any open issues?

Checklist:

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

[cyberj0g]

LGTM overall, left a few minor comments. +1 for nicely-sized PR.

PS: TestParseAcceptsEitherDatadirCasing is failing, is it related to changes?

[codecov]

Codecov Report

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

❗ 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%)	⬇️
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%)	⬇️
drivers/gssave.go	0.00000% <0.00000%> (ø)
pm/stub.go	56.98113% <0.00000%> (-0.43333%)	⬇️
server/ot_rpc.go	35.75581% <19.23077%> (+9.89910%)	⬆️
server/broadcast.go	77.32975% <33.33333%> (-0.58880%)	⬇️
pm/recipient.go	89.63964% <57.14286%> (-2.45338%)	⬇️
server/handlers.go	53.51598% <73.91304%> (+0.43761%)	⬆️
... and 7 more

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