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/fakeconns_test.go

@@ -34,6 +34,7 @@ func newFakeConnection(id protocol.DeviceID, model Model) *fakeConnection {
 		return f.fileData[name], nil
 	})
 	f.DeviceIDReturns(id)
+	f.ConnectionIDReturns(rand.String(16))
 	f.CloseCalls(func(err error) {
 		f.closeOnce.Do(func() {
 			close(f.closed)