Tcp buffer bug fix (#79)

* start working on tracking down tcp buffer bug

* mtu refactor

* modularize test

* more precise testing

* tests fail, but is it the failure we are looking for?

* fix typo in espradio link (#76)

* implement a new backoff abstraction (#75)

* rewrite backoff api

* rewrite tcp.Conn.Write

* keep fixing small things

* much better Conn.Read implementation

* fix critical overflow bug in internal.ConnRWBackoff

---------

Co-authored-by: Joel Wetzell <jwetzell@yahoo.com>

x/xnet/xnet_dns_test.go

@@ -14,7 +14,7 @@ import (
 
 func TestDNS_QueryReceivesAnswer(t *testing.T) {
 	const seed = 9876
-	const MTU = 1500
+	const MTU = ethernet.MaxMTU
 
 	// Create client stack with DNS server configured.
 	client := new(StackAsync)
@@ -47,7 +47,7 @@ func TestDNS_QueryReceivesAnswer(t *testing.T) {
 	}
 
 	// Client sends DNS query.
-	const carrierDataSize = MTU + ethernet.MaxOverheadSize
+	const carrierDataSize = ethernet.MaxFrameLength
 	var buf [carrierDataSize]byte
 	n, err := client.EgressEthernet(buf[:])
 	if err != nil {