message: log, timer: add LOG_TIME_MILLIS_WITH_CATEGORY_MSG_ONCE macro
  that prints the descriptive message when logging the start
  but not when logging the completion.
diff: @@ -100,6 +100,8 @@ private:
  BCLog::Timer<std::chrono::microseconds> PASTE2(logging_timer, __COUNTER__)(__func__, end_msg, log_category)
  #define LOG_TIME_MILLIS_WITH_CATEGORY(end_msg, log_category) \
  BCLog::Timer<std::chrono::milliseconds> PASTE2(logging_timer, __COUNTER__)(__func__, end_msg, log_category)
  +#define LOG_TIME_MILLIS_WITH_CATEGORY_MSG_ONCE(end_msg, log_category) \
  + BCLog::Timer<std::chrono::milliseconds> PASTE2(logging_timer, __COUNTER__)(__func__, end_msg, log_category, /* msg_on_completion=*/false)
  #define LOG_TIME_SECONDS(end_msg) \
  BCLog::Timer<std::chrono::seconds> PASTE2(logging_timer, __COUNTER__)(__func__, end_msg)
  