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.cpp

@@ -0,0 +1,12 @@
+#include <utils/time.hpp>
+
+namespace utils
+{
+std::string to_string(const std::time_t& timestamp)
+{
+    constexpr std::size_t stamp_size = 20;
+    char date_buf[stamp_size];
+    std::strftime(date_buf, stamp_size, "%FT%TZ", std::gmtime(&timestamp));
+    return {std::begin(date_buf), std::end(date_buf)};
+}
+}

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);
+}