Internal Feature: Single, global CONFIG instance

[Tokarak]

Right now, it is difficult to transfer variables (i.e config values) between modules. It would be nice to have a single, extendable, immutable instance of a Config struct which is filled out (in order) from defaults, config file, environment, and command-line flags; and is accessible globally from any module. This in fact is, I believe, the standard deserialisation pattern of Clap.

• Migrate to Clap deserialisation pattern
• Combine all runtime config from all sources into a single struct
  The following could follow
• Refactor instance_info to be generated programmatically
  • Distinguish "public" config values which should be displayed from "internal" ones (such as listening address and port)