Keep the storage alive in Router group management.

When initializing or refreshing the routing groups, keep the storage reference
alive so that we won't re-initialize it for every group we got from the group
lists.
Don't cache routing groups as it's impossible to

update routing groups without cleaning cache by hands.
Get rid of group_t struct.
Merge branch 'noxiouz-disable_group_cache'
Drop unnecessary public typedef.