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/folder_recvonly_test.go

@@ -530,7 +530,7 @@ func setupROFolder(t *testing.T) (*testModel, *receiveOnlyFolder, context.Cancel
 	cfg.Folders = []config.FolderConfiguration{fcfg}
 	replace(t, w, cfg)
 
-	m := newModel(t, w, myID, "syncthing", "dev", nil)
+	m := newModel(t, w, myID, nil)
 	m.ServeBackground()
 	<-m.started
 	must(t, m.ScanFolder("ro"))