Avoid dependnecies or make them optional

[shesek]

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