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

x/xnet/tcppool.go

@@ -80,7 +80,6 @@ func NewTCPPool(cfg TCPPoolConfig) (*TCPPool, error) {
 			TxBuf:             bufSpace[txOff : txOff+cfg.TxBufSize],
 			TxPacketQueueSize: cfg.QueueSize,
 			Logger:            cfg.ConnLogger,
-			NanoTime:          cfg.NanoTime,
 		})
 		if err != nil {
 			return nil, err