Respond to MAM requests on a channel JID

At the moment, result-set-management is not implemented, the whole history
(well, at most 1024 messages) is returned.

louloulibs/utils/time.hpp

@@ -0,0 +1,9 @@
+#pragma once
+
+#include <ctime>
+#include <string>
+
+namespace utils
+{
+std::string to_string(const std::time_t& timestamp);
+}