Refactor Logging Impl

Loop on stop_token and remove final_entry in Entry.
Move Backend thread out of Impl Constructor to its own function.
Add Start function for backend thread.
Use stop token in PopWait and check if entry filename is nullptr before logging.

src/common/logging/backend.h

@@ -14,6 +14,8 @@ class Filter;
 /// Initializes the logging system. This should be the first thing called in main.
 void Initialize();
 
+void Start();
+
 void DisableLoggingInTests();
 
 /**