Don't cache routing groups.

It's impossible to update routing groups without cleaning cache by hands (i.e. `cocaine-tool group refresh` is useless and does really nothing).

May I get rid of `group_t::name` as it isn't used anywhere?

Also I find keeping pointer to the storage alive by hands weird. Is it better to implement a special class `groups_storage`, which would incapsulate `list/read` methods and hold pointer to the storage?