tcp: retransmit logic rollback (#61)

* tcp: remove retransmit logic entirely; add duplicate ack counting to ControlBlock

* retransmit implemented in nice simple straightforward way

* narrow down retransmission cases

* remove old timing tests

* add ControlBlock retransmit test

* add failing handler test

* reworking payload length semantic meaning in code

* fix establish conn logic

* clean up tests and add TCB dupack generation and test it

* catch pending retransmit satisfy in test

* add fuzz test for control block

* bugfix: be more strict in what is considered dupack

* add IncomingIsDupACK docs

* limit queue of retransmits

* protect retransmit overflow from incorrectly updating nxt

internal/prand.go

@@ -17,3 +17,11 @@ func Prand32[T ~uint32](seed T) T {
 	seed ^= seed << 5
 	return seed
 }
+
+// Prand32 generates a pseudo random number from a seed.
+func Prand64[T ~uint64](seed T) T {
+	seed ^= seed << 13
+	seed ^= seed >> 7
+	seed ^= seed << 17
+	return seed
+}