[MEDIUM] further improve monotonic clock by check forward jumps

[MEDIUM] further improve monotonic clock by check forward jumps

The first implementation of the monotonic clock did not verify
forward jumps. The consequence is that a fast changing time may
expire a lot of tasks. While it does seem minor, in fact it is
problematic because most machines which boot with a wrong date
are in the past and suddenly see their time jump by several
years in the future.

The solution is to check if we spent more apparent time in
a poller than allowed (with a margin applied). The margin
is currently set to 1000 ms. It should be large enough for
any poll() to complete.

Tests with randomly jumping clock show that the result is quite
accurate (error less than 1 second at every change of more than
one second).