add and use configuration write guard

the configuration write guard is now required when the configuration
struct shall be mutated. the write guards locks multiple writers against
each other and also, more importantly, makes the writes synchronous to
the main loop. all code running in the main loop can now be sure that
(1) reads from the configuration struct are non-preemtive and (2) the
configuration struct as a whole is in a consistent state when reading
from it.

NOTE that acquiring a write guard from within the main loop's task will
immediately cause a deadlock and the watchdog will trigger a reset. if
writing from inside the main loop should ever become necessary, the
write guard must be updated to only lock the mutex but not wait for a
signal.

include/Configuration.h

@@ -3,6 +3,9 @@
 
 #include "PinMapping.h"
 #include <cstdint>
+#include <TaskSchedulerDeclarations.h>
+#include <mutex>
+#include <condition_variable>
 
 #define CONFIG_FILENAME "/config.json"
 #define CONFIG_VERSION 0x00011c00 // 0.1.28 // make sure to clean all after change
@@ -161,15 +164,32 @@ struct CONFIG_T {
 
 class ConfigurationClass {
 public:
-    void init();
+    void init(Scheduler& scheduler);
     bool read();
     bool write();
     void migrate();
-    CONFIG_T& get();
+    CONFIG_T const& get();
+
+    class WriteGuard {
+    public:
+        WriteGuard();
+        CONFIG_T& getConfig();
+        ~WriteGuard();
+
+    private:
+        std::unique_lock<std::mutex> _lock;
+    };
+
+    WriteGuard getWriteGuard();
 
     INVERTER_CONFIG_T* getFreeInverterSlot();
     INVERTER_CONFIG_T* getInverterConfig(const uint64_t serial);
     void deleteInverterById(const uint8_t id);
+
+private:
+    void loop();
+
+    Task _loopTask;
 };
 
 extern ConfigurationClass Configuration;