pair of lower/upper bound iterators.

Sometimes when iterating the list, like in cs_mode, we can modify the
contents of it, which combined with mlock always agressively trying to
readd modes to it can do bad things.

Cache xline nick, user, host, etc instead of rebuilding it everytime its
requested. Store users ip in sockaddr form and not string form to
prevent having to rebuild sockaddrs when checking xlines.

Also do not try to convert empty config values in Config::Get as this
can be rather common if a non string configuration value is not set, and
the cost of the ConvertException is great.