lib/protocol, lib/discover, lib/db: Use protocol buffer serialization (fixes #3080)

This changes the BEP protocol to use protocol buffer serialization
instead of XDR, and therefore also the database format. The local
discovery protocol is also updated to be protocol buffer format.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3276
LGTM: AudriusButkevicius

lib/connections/service.go

@@ -162,8 +162,17 @@ next:
 		if err != nil {
 			if protocol.IsVersionMismatch(err) {
 				// The error will be a relatively user friendly description
-				// of what's wrong with the version compatibility
-				msg := fmt.Sprintf("Connecting to %s (%s): %s", remoteID, c.RemoteAddr(), err)
+				// of what's wrong with the version compatibility. By
+				// default identify the other side by device ID and IP.
+				remote := fmt.Sprintf("%v (%v)", remoteID, c.RemoteAddr())
+				if hello.DeviceName != "" {
+					// If the name was set in the hello return, use that to
+					// give the user more info about which device is the
+					// affected one. It probably says more than the remote
+					// IP.
+					remote = fmt.Sprintf("%q (%s %s, %v)", hello.DeviceName, hello.ClientName, hello.ClientVersion, remoteID)
+				}
+				msg := fmt.Sprintf("Connecting to %s: %s", remote, err)
 				warningFor(remoteID, msg)
 			} else {
 				// It's something else - connection reset or whatever