[TLDK-2] TLDK does not send RST

[vvalderrv]

When an unknown packet arrives (i.e. - a packet that is destined for a non-existing stream, or for a port where noone is listening), conforming TCP implementations send out a reset packet. TLDK does not do that.

If chaining of packets between different contexts is necessary (is it?), then we can have an optional flag saying "reject unknown packets". When that is set, any packet that does not belong to any stream should be rejected with a corresponding RST packet (or RST+ACK, if in response to a SYN).