MXS-2555 Simple Smart Router

Does the measurments, usage of the same  and persistence to file.
Kill is not implemented, so waits for all clusters to fully responds

The performance data uses a mutex and the persistence data file
is written while holding the mutex. This obviously needs to be
improved, but this commit shows the working concept.

server/modules/routing/smartrouter/performance.hh

@@ -0,0 +1,117 @@
+/*
+ * Copyright (c) 2019 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: 2022-01-01
+ *
+ * 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 <maxscale/ccdefs.hh>
+
+#include <maxbase/stopwatch.hh>
+#include <maxbase/host.hh>
+
+#include <unordered_map>
+
+/** Class PerformanceInfo is a basic structure for storing a Host and Duration pair, along with
+ *  the time it was created.
+ */
+class PerformanceInfo
+{
+public:
+    PerformanceInfo() = default;    // creates an instance where is_valid()==false;
+    PerformanceInfo(const maxbase::Host& h, maxbase::Duration d);
+
+    bool is_valid() const;
+
+    maxbase::Host     host() const;
+    maxbase::Duration duration() const;
+
+    /** When was this PerformanceInfo created.
+     */
+    maxbase::TimePoint creation_time() const;
+    /** Duration since this PerformanceInfo was created
+     */
+    maxbase::Duration age() const;
+private:
+    maxbase::Host     m_host;
+    maxbase::Duration m_duration;
+
+    maxbase::TimePoint m_creation_time = maxbase::Clock::now();
+};
+
+/** class CanonicalPerformance holds the persistent performance
+ *  info gathered thus far.
+ */
+class CanonicalPerformance
+{
+public:
+    CanonicalPerformance(const std::string& persistent_file);
+
+    /** Insert if not already inserted and return true, else false. */
+    bool insert(const std::string& canonical, const PerformanceInfo& perf);
+
+    /** Remove if entry exists and return true, else false. */
+    bool remove(const std::string& canonical);
+
+    /** If entry does not exists, returned PerformanceInfo::is_valid()==false */
+    PerformanceInfo find(const std::string& canonical);
+
+    void clear();
+    void persist() const;
+
+private:
+    const std::string                                m_persistent_file;
+    std::unordered_map<std::string, PerformanceInfo> m_perfs;
+    void read_persisted();
+    mutable int m_nChanges;
+};
+
+// Threadsafe global singleton behind perf_find and perf_update.
+// perf_find, find existing performance info, handles expirations
+PerformanceInfo perf_find(const std::string& canonical);
+
+// Insert if not already inserted and return true, else false.
+// Will probably handle some expiration like work.
+bool perf_update(const std::string& canonical, const PerformanceInfo& perf);
+
+// For logging. Shortens str to nchars and adds "..." TODO move somewhere more appropriate
+std::string show_some(const std::string& str, int nchars = 70);
+
+// implementation details below
+inline PerformanceInfo::PerformanceInfo(const maxbase::Host& h, maxbase::Duration d)
+    : m_host(h)
+    , m_duration(d)
+{
+}
+
+inline bool PerformanceInfo::is_valid() const
+{
+    return m_host.is_valid();
+}
+
+inline maxbase::Host PerformanceInfo::host() const
+{
+    return m_host;
+}
+
+inline maxbase::Duration PerformanceInfo::duration() const
+{
+    return m_duration;
+}
+
+inline maxbase::TimePoint PerformanceInfo::creation_time() const
+{
+    return m_creation_time;
+}
+
+inline maxbase::Duration PerformanceInfo::age() const
+{
+    return maxbase::Clock::now() - m_creation_time;
+}