logging/backend: Make time_origin a class variable instead of a local static

Moves local global state into the Impl class itself and initializes it
at the creation of the instance instead of in the function.

This makes it nicer for weakly-ordered architectures, given the
CreateEntry() class won't need to have atomic loads executed for each
individual call to the CreateEntry class.

src/common/logging/backend.cpp

@@ -115,8 +115,6 @@ private:
         using std::chrono::duration_cast;
         using std::chrono::steady_clock;
 
-        static steady_clock::time_point time_origin = steady_clock::now();
-
         Entry entry;
         entry.timestamp =
             duration_cast<std::chrono::microseconds>(steady_clock::now() - time_origin);
@@ -135,6 +133,7 @@ private:
     std::vector<std::unique_ptr<Backend>> backends;
     Common::MPSCQueue<Log::Entry> message_queue;
     Filter filter;
+    std::chrono::steady_clock::time_point time_origin{std::chrono::steady_clock::now()};
 };
 
 void ConsoleBackend::Write(const Entry& entry) {