Monotonic clock support

It would be really nice if we could make Mojolicious more resilient to time jumps with monotonic clock support. This pull request is not quite ready yet, but should be rather easy to finish once the patch for OS X support has been applied to Time::HiRes.

https://rt.cpan.org/Public/Bug/Display.html?id=78656

The EV reactor already uses a monotonic clock internally (through libev), but there is no public API, which we would need to make timeouts work properly on all layers.