Accessing the same LDAP* from multiple threads at once is always
undefined, even if one thread is just polling ldap_result.

Instead keep one thread per connection and issue blocking queries on the
thread.

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.