Introduce support for asynchronous server-side methods (#12)

* Add preliminary changes for async server methods

* Refactor the Message concept and break it into distinctive types

* Continue working on async server methods (high-level API mainly)

* Continue developing support for async server

* Finishing async server methods

* Finishing async server methods (fixing tests & cleaning up)

* A little code cleaning

* Add unit tests for type traits of free functions

* Support for generating async server methods in stub headers

* Update ChangeLog for v0.3.0

* Update the tutorial with how to use async server-side methods

* Update the TOC in sdbus-c++ tutorial

* Update numbering in TOC

* Remove unnecessary code

* Final cleanups

test/integrationtests/TestingAdaptor.h

@@ -27,6 +27,8 @@
 #define SDBUS_CPP_INTEGRATIONTESTS_TESTINGADAPTOR_H_
 
 #include "adaptor-glue.h"
+#include <thread>
+#include <chrono>
 
 class TestingAdaptor : public sdbus::Interfaces<testing_adaptor>
 {
@@ -97,6 +99,22 @@ protected:
         return res;
     }
 
+    uint32_t doOperationSync(uint32_t param)
+    {
+        std::this_thread::sleep_for(std::chrono::milliseconds(param));
+        return param;
+    }
+
+    void doOperationAsync(uint32_t param, sdbus::Result<uint32_t> result)
+    {
+        // The same as doOperationSync, just written as an asynchronous method callback
+        std::thread([param, result = std::move(result)]()
+        {
+            std::this_thread::sleep_for(std::chrono::milliseconds(param));
+            result.returnResults(param);
+        }).detach();
+    }
+
     sdbus::Signature getSignature() const { return SIGNATURE_VALUE; }
     sdbus::ObjectPath getObjectPath() const { return OBJECT_PATH_VALUE; }