We have no explicit signal when a UDP stream has terminated, peers just
stop sending.

For suspected stream connections a timeout of two minutes is sane to keep
NAT mapping alive a while longer.

It matches tcp conntracks 'timewait' default timeout value.
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>