Fix sporadic race condition between Variant and underlying bus

The underlying bus was thread_local, but the design assumption that Variants built on top of that instance won't outlive the thread was incorrect. In stress tests, Variants were moved (and this is completely legal) to a different thread.

src/ISdBus.h

@@ -51,6 +51,7 @@ namespace sdbus { namespace internal {
         virtual int sd_bus_call(sd_bus *bus, sd_bus_message *m, uint64_t usec, sd_bus_error *ret_error, sd_bus_message **reply) = 0;
         virtual int sd_bus_call_async(sd_bus *bus, sd_bus_slot **slot, sd_bus_message *m, sd_bus_message_handler_t callback, void *userdata, uint64_t usec) = 0;
 
+        virtual int sd_bus_message_new(sd_bus *bus, sd_bus_message **m, uint8_t type) = 0;
         virtual int sd_bus_message_new_method_call(sd_bus *bus, sd_bus_message **m, const char *destination, const char *path, const char *interface, const char *member) = 0;
         virtual int sd_bus_message_new_signal(sd_bus *bus, sd_bus_message **m, const char *path, const char *interface, const char *member) = 0;
         virtual int sd_bus_message_new_method_return(sd_bus_message *call, sd_bus_message **m) = 0;