reftable/stack: stop using `sleep_millisec()`

Refactor our use of `sleep_millisec()` by open-coding it with poll(3p),
which is the current implementation of this function. Ideally, we'd use
a more direct way to sleep, but there is no equivalent to sleep(3p) that
would accept milliseconds as input.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

reftable/stack.c

@@ -524,7 +524,7 @@ static int reftable_stack_reload_maybe_reuse(struct reftable_stack *st,
 		fd = -1;
 
 		delay = delay + (delay * reftable_rand()) / UINT32_MAX + 1;
-		sleep_millisec(delay);
+		poll(NULL, 0, delay);
 	}
 
 out: