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

tcp/definitions.go

@@ -172,6 +172,8 @@ const (
 	synack = FlagSYN | FlagACK
 	finack = FlagFIN | FlagACK
 	pshack = FlagPSH | FlagACK
+
+	flagctl = FlagSYN | FlagFIN | FlagRST
 )
 
 // HasAll checks if mask bits are all set in the receiver flags.