lib/protocol: Use DeviceID in protocol messages, with custom marshalling

This makes the device ID a real type that can be used in the protobuf
schema. That avoids the juggling back and forth from []byte in a bunch
of places and simplifies the code.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3695

lib/protocol/deviceid_test.proto

@@ -0,0 +1,20 @@
+
+syntax = "proto3";
+
+package protocol;
+
+import "github.com/gogo/protobuf/gogoproto/gogo.proto";
+
+option (gogoproto.goproto_getters_all) = false;
+option (gogoproto.sizer_all) = false;
+option (gogoproto.protosizer_all) = true;
+option (gogoproto.goproto_enum_stringer_all) = false;
+option (gogoproto.goproto_enum_prefix_all) = false;
+
+message TestOldDeviceID {
+    bytes test = 1;
+}
+
+message TestNewDeviceID {
+    bytes test = 1 [(gogoproto.customtype) = "DeviceID", (gogoproto.nullable) = false];
+}