(TK-58) Add remaining default server support

This PR addresses a bug wherein `get-registered-endpoints`, `log-registered-endpoints`, `join`, and `override-webserver-settings!` were using a default server with a key of `:default` rather than the actual default server specified by the user. 
It wasn't using a multiserver configuration before, so the `:default` key was a valid server, since a single-server configuration will assign the single server an id of `:default`.