Refactor livepeer.go to enable running Livepeer node from the code

[leszko]

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

Refactor livepeer.go to allow running Livepeer node from the code. After this change, you can start Livepeer node from the code (without passing any flags) in the following manner.

// Start Livepeer node
go func() {
        // Sample configuration for combined O/T topology
	datadir := "~/temp/livepeer/TEST_OT"
	orchestrator := true
	transcoder := true
	serviceAddr := "127.0.0.1:8935"
	cfg := DefaultLivepeerConfig()
	cfg.datadir, cfg.orchestrator, cfg.transcoder, cfg.serviceAddr = &datadir, &orchestrator, &transcoder, &serviceAddr

        // Starting the Livepeer node
	StartLivepeer(ctx, cfg)
}()

The context for this change is the work on implementing E2E Testing which would cover interactions with the chain. Additionally, I believe it's a way better code structure when we split the configuration and the flags parsing.

Some ideas for future improvements (not included in this PR):

• Allow running Livepeer as library for end-users (improve LivepeerConfig structure)
• Refactor main() to extract it into functions like parseOrchAddrs()
• Make sure it's possible to run multiple Livepeer instances in one process (in general it works, but may need some tweaks)

Specific updates (required)

• Create the LivepeerConfig type and the DefaultLivepeerConfig() function
• Change parsing flags to fill LivepeerConfig
• Set nils in LivepeerConfig instead of using the isFlagSet map
• Extract parseOrchAddrs() function

How did you test each of these updates (required)

Started Livepeer on-chain and off-chain and played with transcoding and CLI interactions.

Does this pull request close any open issues?

No, but it's related to #2317

Checklist:

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

[thomshutt]

Looks good, I like the ideas for future steps too

[thomshutt]

👍 I've always found it annoying that the Go flags package doesn't give a way to distinguish between zero value / not set, but good to at least do it all in one place