refs #601 Simplify syncLoad and rename to synchronize.

Now it doesn't return anything, it just causes the next async get() to be synchronized with the latest loaded value.
It does this by hooking into the queue introduced in the previous commit.

src/blackmisc/valuecache.cpp

@@ -74,6 +74,14 @@ namespace BlackMisc
         return result;
     }
 
+    CValueCachePacket CValueCachePacket::takeByKey(const QString &key)
+    {
+        auto copy = *this;
+        remove(key);
+        copy.removeByKeyIf([ = ](const QString &key2) { return key2 != key; });
+        return copy;
+    }
+
     void CValueCachePacket::registerMetadata()
     {
         MetaType::registerMetadata();