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/model/requests_test.go

@@ -1214,7 +1214,7 @@ func TestRequestIndexSenderClusterConfigBeforeStart(t *testing.T) {
 
 	// Initialise db with an entry and then stop everything again
 	must(t, tfs.Mkdir(dir1, 0o777))
-	m := newModel(t, w, myID, "syncthing", "dev", nil)
+	m := newModel(t, w, myID, nil)
 	defer cleanupModelAndRemoveDir(m, tfs.URI())
 	m.ServeBackground()
 	m.ScanFolders()
@@ -1223,7 +1223,7 @@ func TestRequestIndexSenderClusterConfigBeforeStart(t *testing.T) {
 
 	// Add connection (sends incoming cluster config) before starting the new model
 	m = &testModel{
-		model:    NewModel(m.cfg, m.id, m.clientName, m.clientVersion, m.db, m.protectedFiles, m.evLogger, protocol.NewKeyGenerator()).(*model),
+		model:    NewModel(m.cfg, m.id, m.db, m.protectedFiles, m.evLogger, protocol.NewKeyGenerator()).(*model),
 		evCancel: m.evCancel,
 		stopped:  make(chan struct{}),
 	}