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_sendrecv.go

@@ -507,7 +507,7 @@ nextFile:
 
 		devices := snap.Availability(fileName)
 		for _, dev := range devices {
-			if _, ok := f.model.Connection(dev); ok {
+			if f.model.ConnectedTo(dev) {
 				// Handle the file normally, by copying and pulling, etc.
 				f.handleFile(fi, snap, copyChan)
 				continue nextFile