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/connections/registry/registry.go

@@ -12,6 +12,7 @@ package registry
 import (
 	"strings"
 
+	"github.com/syncthing/syncthing/lib/sliceutil"
 	"github.com/syncthing/syncthing/lib/sync"
 )
 
@@ -41,9 +42,7 @@ func (r *Registry) Unregister(scheme string, item interface{}) {
 	candidates := r.available[scheme]
 	for i, existingItem := range candidates {
 		if existingItem == item {
-			candidates[i] = candidates[len(candidates)-1]
-			candidates[len(candidates)-1] = nil
-			r.available[scheme] = candidates[:len(candidates)-1]
+			r.available[scheme] = sliceutil.RemoveAndZero(candidates, i)
 			break
 		}
 	}