Avoid dependnecies or make them optional

[shesek]

• [x] structopt (cli only, used for parsing arguments)
• [x] dotenv (cli only, used for config file)
• [x] dirs (cli only or enabled with extra, provides auto-detection of the bitcoin datadir)
• [x] pretty_env_logger (cli only or enabled with extra, used for logging)
• [x] signal-hook (cli only or enabled with extra, used to handle SIGINT/SIGTERM signals)
• [x] hex (removed)
• [ ] chrono (also used for the welcome banner apart from the cli, but this could be avoided)
• [ ] lazy_static