Replace the boost::condition_variable used in fc's task scheduler

... with a std::condition_variable. The boost::condition_variable's wait_for/ wait_until methods misbehaved when run under faketime --exclude-monotonic, which we are using in our CI tests. That would cause any thread with a task scheduled in the future to busy-wait for the scheduled time to arrive instead of sleeping. std::condition_variables don't have this problem.