all: Support multiple device connections (fixes #141) (#8918)

This adds the ability to have multiple concurrent connections to a single device. This is primarily useful when the network has multiple physical links for aggregated bandwidth. A single connection will never see a higher rate than a single link can give, but multiple connections are load-balanced over multiple links.

It is also incidentally useful for older multi-core CPUs, where bandwidth could be limited by the TLS performance of a single CPU core -- using multiple connections achieves concurrency in the required crypto calculations...

Co-authored-by: Simon Frei <freisim93@gmail.com>
Co-authored-by: tomasz1986 <twilczynski@naver.com>
Co-authored-by: bt90 <btom1990@googlemail.com>

lib/protocol/hello.go

@@ -8,14 +8,6 @@ import (
 	"io"
 )
 
-// The HelloIntf interface is implemented by the version specific hello
-// message. It knows its magic number and how to serialize itself to a byte
-// buffer.
-type HelloIntf interface {
-	Magic() uint32
-	Marshal() ([]byte, error)
-}
-
 var (
 	// ErrTooOldVersion is returned by ExchangeHello when the other side
 	// speaks an older, incompatible version of the protocol.
@@ -25,7 +17,10 @@ var (
 	ErrUnknownMagic = errors.New("the remote device speaks an unknown (newer?) version of the protocol")
 )
 
-func ExchangeHello(c io.ReadWriter, h HelloIntf) (Hello, error) {
+func ExchangeHello(c io.ReadWriter, h Hello) (Hello, error) {
+	if h.Timestamp == 0 {
+		panic("bug: missing timestamp in outgoing hello")
+	}
 	if err := writeHello(c, h); err != nil {
 		return Hello{}, err
 	}
@@ -80,7 +75,7 @@ func readHello(c io.Reader) (Hello, error) {
 	return Hello{}, ErrUnknownMagic
 }
 
-func writeHello(c io.Writer, h HelloIntf) error {
+func writeHello(c io.Writer, h Hello) error {
 	msg, err := h.Marshal()
 	if err != nil {
 		return err