[PERF] Use std::call_once to arrange timer record stop

[dmantipov]

Prefer std::once_flag and std::call_once() over plain C-style flag to arrange (the body of) TimerRecord::stop() to execute just once, adjust related code.

Signed-off-by: Dmitry Antipov [email protected]

[rui314]

std::call_once is used to make the thing that is protected by call_once thread-safe. In this case, stop is called by a single thread, so we don't need to use call_once.