tproxy: added const specifiers to udp lookup functions

The parameters for various UDP lookup functions were non-const, even though
they could be const. TProxy has some const references and instead of
downcasting it, I added const specifiers along the path.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
Signed-off-by: KOVACS Krisztian <hidden@balabit.hu>
Signed-off-by: Patrick McHardy <kaber@trash.net>