MXS-173 Test for throttling filter

The test may fail if the client/maxscale/mariadb combo is too slow.
TODO, maybe: today I saw mysql_query() hang (in a poll) when maxscale
disconnected. Is there something that can be done about that?
I added a source directory, base, for stuff that should become part of a common
utility library in the future.

maxscale-system-test/base/stopwatch.h

@@ -0,0 +1,56 @@
+/*
+ * Copyright (c) 2018 MariaDB Corporation Ab
+ *
+ * Use of this software is governed by the Business Source License included
+ * in the LICENSE.TXT file and at www.mariadb.com/bsl11.
+ *
+ * Change Date: 2020-01-01
+ *
+ * On the date above, in accordance with the Business Source License, use
+ * of this software will be governed by version 2 or later of the General
+ * Public License.
+ */
+#pragma once
+
+#include <chrono>
+#include <iosfwd>
+#include <string>
+
+// Same StopWatch as in maxscale, but dropping support for gcc 4.4
+namespace base
+{
+using Clock = std::chrono::steady_clock;
+
+struct Duration : public Clock::duration // for ADL
+{
+    using Clock::duration::duration;
+    Duration() = default;
+    Duration(Clock::duration d) : Clock::duration(d) {}
+};
+
+using TimePoint = std::chrono::time_point<Clock, Duration>;
+
+class StopWatch
+{
+public:
+    // Starts the stopwatch, which is always running.
+    StopWatch();
+    // Get elapsed time.
+    Duration lap() const;
+    // Get elapsed time, restart StopWatch.
+    Duration restart();
+private:
+    TimePoint m_start;
+};
+
+// Returns the value as a double and string adjusted to a suffix like ms for milliseconds.
+std::pair<double, std::string> dur_to_human_readable(Duration dur);
+
+// Human readable output. No standard library for it yet.
+std::ostream& operator<<(std::ostream&, Duration dur);
+
+// TimePoint
+std::string time_point_to_string(TimePoint tp, const std::string& fmt = "%F %T");
+std::ostream& operator<<(std::ostream&, TimePoint tp);
+
+} // base