refactor: use sd-bus API to get current message

src/SdBus.h

@@ -79,6 +79,7 @@ public:
     virtual int sd_bus_start(sd_bus *bus) override;
 
     virtual int sd_bus_process(sd_bus *bus, sd_bus_message **r) override;
+    virtual sd_bus_message* sd_bus_get_current_message(sd_bus *bus) override;
     virtual int sd_bus_get_poll_data(sd_bus *bus, PollData* data) override;
     virtual int sd_bus_get_n_queued_read(sd_bus *bus, uint64_t *ret) override;
     virtual int sd_bus_flush(sd_bus *bus) override;