Logging setup and cli arg handling could be cleaner

[metachris]

Logging and CLI arg handling is a bit hacky.

For instance:

• some vars defined here and reused: https://github.com/flashbots/relayscan/blob/3a3dcb24167078f66952ad41034124360ba6bbec/cmd/service/service.go#L10
• logging actually instantiated here: https://github.com/flashbots/relayscan/blob/3a3dcb24167078f66952ad41034124360ba6bbec/common/logging.go

Improvements:

• Switch to https://github.com/urfave/cli for CLI argument handling
• Switch to built-in logger (see also https://github.com/flashbots/go-template/blob/main/cmd/cli/main.go#L26)