time: separate sleep time calculation from sleep_for method

We need to replace nanosleeps with condition variables so that we
can implement BeesContext::stop.  Export the time calculation from
sleep_for() into a new method called sleep_time().

If the thread executing RateLimiter::sleep_for() is interrupted, it will
no longer be able to restart, as the sleep_time() method is destructive.
This calls for further refactoring of sleep_time() into destructive
and non-destructive parts; however, there are currently no users of
sleep_for() which rely on being able to restart after being interrupted
by a signal.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

include/crucible/time.h

@@ -42,6 +42,7 @@ namespace crucible {
 		RateLimiter(double rate, double burst);
 		RateLimiter(double rate);
 		void sleep_for(double cost = 1.0);
+		double sleep_time(double cost = 1.0);
 		bool is_ready();
 		void borrow(double cost = 1.0);
 	};