bpo-34819: Use a monotonic clock to compute timeouts in concurrent.futures (GH-9599)

Use a monotonic clock to compute timeouts in :meth:`Executor.map` and :func:`as_completed`, in order to prevent timeouts from deviating when the system clock is adjusted.

This may not be sufficient on all systems. On POSIX for example, the actual waiting (e.g. in ``sem_timedwait``) is specified to rely on the CLOCK_REALTIME clock.