Use mtime to measure times

[talex5]

Using mtime has several advantages:

• It isn't affected by changes to the system clock.
• It has better precision.
• Anything using Prometheus_app will have working time (without having to remember to call Prometheus.init itself).
  Since Prometheus_app includes the text formatter, this should be most things.

(When releasing v1.4, it turned out that using Prometheus_app without Prometheus_unix was more common than I expected in #68)

[talex5]

With this change, OCurrent and OCluster tests pass without needing any changes.

[talex5]

Actually, does mtime.clock work in unikernels these days? It says it works with js_of_ocaml, but maybe it doesn't work in general?

(e.g. mini-os has uint64_t monotonic_clock(void) and solo5 has solo5_time_t solo5_clock_monotonic(void), but I'm not sure if mtime knows about them)