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.