Add support for hot reloading of configuration

[lanzafame]

As many config values should be hot reloadable as possible, i.e. cache sizes, path values, etc. The config file doesn't needed to watched for changes but should be reprocessed upon receiving a signal, i.e. SIGHUP. This would bring iroh inline with web2 web servers like nginx.

[fabricedesre]

In addition to reloading when receiving a signal, it would be useful to also be change the configuration through an rpc command.

[ramfox]

This fits into a larger discussion we need to have about how we expect users to interact with the config, and how much help the command line tools are expected to provide. At minimum we need some clear docs on our docs site about how to use our config files.