update currentRound endpoint to return string instead of bytes

[ad-astra-video]

What does this pull request do? Explain your changes. (required)
Updates currentRound web endpoint to return a string rather than big.Int.Bytes().

Makes it easier to pull from livepeer node by curl request and outside call to web endpoint.

Specific updates (required)

• currentRound handler updated to return string instead of big.Int.Bytes()
• livepeer_cli updated to handle new return value
• test updated

How did you test each of these updates (required)
ran test suite on fork of go-livepeer https://github.com/ad-astra-video/go-livepeer/actions/runs/6109883984

Does this pull request close any open issues?
No

Checklist:

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

[codecov]

Codecov Report

Merging #2859 (cf67ca5) into master (4a4a956) will increase coverage by 0.00385%.
The diff coverage is 16.66667%.

@@                 Coverage Diff                 @@
##              master       #2859         +/-   ##
===================================================
+ Coverage   56.48625%   56.49010%   +0.00385%     
===================================================
  Files             88          88                 
  Lines          19233       19237          +4     
===================================================
+ Hits           10864       10867          +3     
- Misses          7770        7771          +1     
  Partials         599         599                 

Files Changed	Coverage Δ
cmd/livepeer_cli/wizard_rounds.go	0.00000% <0.00000%> (ø)
server/handlers.go	53.07018% <100.00000%> (ø)

... and 1 file with indirect coverage changes

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Files Changed	Coverage Δ
cmd/livepeer_cli/wizard_rounds.go	0.00000% <0.00000%> (ø)
server/handlers.go	53.07018% <100.00000%> (ø)

... and 1 file with indirect coverage changes

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