rfc(SSOT): Add Single Source of Truth (SSOT) configuration system

[big-r81]

Initial ideas of a Single Source Of Truth config system for CouchDB.

[rnewson]

Before we get too deep into this I'd like to see us tidy up what we currently have. Specifically;

move code defaults to priv config files:

Like stats_descriptions.cfg, for each application create a file called config_defaults.cfg. This file could look like;

src/couch/priv/config_defaults.cfg:

#{
  max_dbs_open => #{
    default => 500,
    description => <<"Maximum number of .couch files to open at once. ...">>
  }
}

These files would replace the commented out items in default.ini. An endpoint could be added to return this data (but not allow them to be modified).

config:get/2 would be enhanced to retrieve the default from these config files (if present, undefined if not) and all uses of config:get/3 would be removed, so that the only copy of the default value for a given config item is in the config_defaults.cfg.

This means default.ini is empty except for things like database_dir with an actual install-specific value (whether from dev/run or a package install. Arguably it could be eliminated entirely and all of those settings could be in local.ini.