Decouple drift counter time of init with usage

This fixes spurious drift warnings when the drift counter was
initialized, but timers that use it not yet started.

Author: halfgaar

driftcounter.cpp

@@ -25,7 +25,7 @@ std::chrono::milliseconds DriftCounter::getDrift() const
 {
     auto now = std::chrono::steady_clock::now();
 
-    const auto last_update_copy = last_update;
+    const auto last_update_copy = last_update.value_or(std::chrono::steady_clock::now());
 
     if (last_update_copy + (std::chrono::milliseconds(HEARTBEAT_INTERVAL) * 2 ) < now)
         return std::chrono::duration_cast<std::chrono::milliseconds>(now - last_update_copy);

driftcounter.h

@@ -3,6 +3,7 @@
 
 #include <chrono>
 #include <array>
+#include <optional>
 
 /**
  * @brief The DriftCounter class allows measuring drift in threads.
@@ -12,7 +13,7 @@
  */
 class DriftCounter
 {
-    std::chrono::time_point<std::chrono::steady_clock> last_update = std::chrono::steady_clock::now();
+    std::optional<std::chrono::time_point<std::chrono::steady_clock>> last_update;
     std::chrono::milliseconds last_drift = std::chrono::milliseconds(0);
     std::array<std::chrono::milliseconds, 16> many_drifts{};
     unsigned int many_index = 0;