Do not stop listening after "ERROR: NAT/TPROXY lookup failed..." (#837)

Do not stop listening after "ERROR: NAT/TPROXY lookup failed..." (#837)

_Intermittent_ NAT/TPROXY lookup failures can happen for several reasons
outside Squid control. For example, flushing conntrack tables under load
is likely to trigger such errors. Squid should not stop listening if
there is a good change that the next accept(2) will be fully successful.
We already apply the same logic to several other accept problems today.

Also added an XXX that TcpAcceptor::mustStop() does not work as expected
on error.