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_test.go

@@ -35,10 +35,11 @@ func TestVersion14Hello(t *testing.T) {
 
 	conn := &readWriter{outBuf, inBuf}
 
-	send := &Hello{
+	send := Hello{
 		DeviceName:    "this device",
 		ClientName:    "other client",
 		ClientVersion: "v0.14.6",
+		Timestamp:     1234567890,
 	}
 
 	res, err := ExchangeHello(conn, send)
@@ -80,10 +81,11 @@ func TestOldHelloMsgs(t *testing.T) {
 
 		conn := &readWriter{outBuf, inBuf}
 
-		send := &Hello{
+		send := Hello{
 			DeviceName:    "this device",
 			ClientName:    "other client",
 			ClientVersion: "v1.0.0",
+			Timestamp:     1234567890,
 		}
 
 		_, err := ExchangeHello(conn, send)