cmd/livepeer: Use price feed watcher for dynamic pricePerPixel

[victorges]

What does this pull request do? Explain your changes. (required)
This uses the Chainlink price feed watcher component created on #2972 to
support dynamic transcode pricing for Broadcasters and Orchestrators. It works
by simply updating the LivepeerNode.SetBasePrice or server.BroadcastCfg.SetMaxPrice
when there are price updates on the price feed.

Specific updates (required)

• Make the price flags strings to support currency specification
• Create new flag for the price feed address
• Start price update loop for Bs and Os

How did you test each of these updates (required)

• go test
• Ran B/O and checked price update happens as expected

Does this pull request close any open issues?
Implements ENG-1726

Checklist:

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

[leszko]

Added some comments. It looks good, but I think it requires some more manual testing. Unfortunately go-livepeer is a beast and we don't have too many automated tests. Therefore, we need to check manually at least the following scenarios:

1. Set up local dev (or ask some public O to test it on Mainnet):

• O: Set different prices per Broadcasters read from a file
• O: Set dynamically prices per Broadcasters
• O: Set dynamically default price from wei to USD and back to wei and back to USD
• B: Set max price from commandline
• B: Set max price dynamically

2. Check if automated tests work in Catalyst (enough to send a draft PR with the update go-livepeer version to your commit hash)

[leszko]

LGTM 👍

[codecov]

Codecov Report

Attention: Patch coverage is 55.41796% with 144 lines in your changes are missing coverage. Please review.

Project coverage is 57.47690%. Comparing base (ef5d789) to head (32c6f14).

Additional details and impacted files

@@                 Coverage Diff                 @@
##              master       #2981         +/-   ##
===================================================
+ Coverage   57.24231%   57.47690%   +0.23459%     
===================================================
  Files             91          92          +1     
  Lines          15513       15695        +182     
===================================================
+ Hits            8880        9021        +141     
- Misses          6040        6078         +38     
- Partials         593         596          +3     

Files	Coverage Δ
cmd/livepeer/livepeer.go	49.66443% <100.00000%> (+0.34011%)	⬆️
core/livepeernode.go	68.18182% <100.00000%> (+5.02393%)	⬆️
server/broadcast.go	80.38793% <100.00000%> (+0.27982%)	⬆️
core/autoconvertedprice.go	93.44262% <93.44262%> (ø)
server/handlers.go	55.74289% <82.97872%> (+0.76408%)	⬆️
cmd/livepeer_cli/wizard_stats.go	0.00000% <0.00000%> (ø)
cmd/livepeer_cli/wizard_broadcast.go	0.00000% <0.00000%> (ø)
eth/watchers/pricefeedwatcher.go	67.30769% <63.26531%> (+26.34383%)	⬆️
cmd/livepeer_cli/wizard_transcoder.go	0.00000% <0.00000%> (ø)
cmd/livepeer/starter/starter.go	7.19352% <30.33708%> (+1.97613%)	⬆️

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Files	Coverage Δ
cmd/livepeer/livepeer.go	49.66443% <100.00000%> (+0.34011%)	⬆️
core/livepeernode.go	68.18182% <100.00000%> (+5.02393%)	⬆️
server/broadcast.go	80.38793% <100.00000%> (+0.27982%)	⬆️
core/autoconvertedprice.go	93.44262% <93.44262%> (ø)
server/handlers.go	55.74289% <82.97872%> (+0.76408%)	⬆️
cmd/livepeer_cli/wizard_stats.go	0.00000% <0.00000%> (ø)
cmd/livepeer_cli/wizard_broadcast.go	0.00000% <0.00000%> (ø)
eth/watchers/pricefeedwatcher.go	67.30769% <63.26531%> (+26.34383%)	⬆️
cmd/livepeer_cli/wizard_transcoder.go	0.00000% <0.00000%> (ø)
cmd/livepeer/starter/starter.go	7.19352% <30.33708%> (+1.97613%)	⬆️