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/sliceutil/sliceutil_test.go

@@ -0,0 +1,28 @@
+// Copyright (C) 2023 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package sliceutil_test
+
+import (
+	"testing"
+
+	"github.com/syncthing/syncthing/lib/sliceutil"
+	"golang.org/x/exp/slices"
+)
+
+func TestRemoveAndZero(t *testing.T) {
+	a := []int{1, 2, 3, 4, 5}
+	b := sliceutil.RemoveAndZero(a, 2)
+	exp := []int{1, 2, 4, 5}
+	if !slices.Equal(b, exp) {
+		t.Errorf("got %v, expected %v", b, exp)
+	}
+	for _, e := range a {
+		if e == 3 {
+			t.Errorf("element should have been zeroed")
+		}
+	}
+}