all: Use context in lib/dialer (#6177)

* all: Use context in lib/dialer

* a bit slimmer

* https://github.com/syncthing/syncthing/pull/5753

* bot

* missed adding debug.go

* errors.Cause

* simultaneous dialing

* anti-leak

lib/dialer/debug.go

@@ -0,0 +1,23 @@
+// Copyright (C) 2019 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 dialer
+
+import (
+	"os"
+	"strings"
+
+	"github.com/syncthing/syncthing/lib/logger"
+)
+
+var (
+	l = logger.DefaultLogger.NewFacility("dialer", "Dialing connections")
+	// To run before init() of other files that log on init.
+	_ = func() error {
+		l.SetDebug("dialer", strings.Contains(os.Getenv("STTRACE"), "dialer") || os.Getenv("STTRACE") == "all")
+		return nil
+	}()
+)