Implement a monotonic time source.

[Yawning]

Go does not currently provide one (See: golang/go#12914). While it is assumed that all mixes will run NTP or similar, the scheduling should be resilient to the system clock changing, thus using time.Now() is unacceptable.

There's a few ways to do this, such as exposing runtime.nanotime() with unsafe trickery, or by calling clock_gettime() since the only OS that this actually matters on is Linux.

[Yawning]

The timer routines in time use runtime.nanotime(), which on Linux is backed by CLOCK_MONOTONIC. As long as the core scheduling only ever uses absolute time based timing things, this shouldn't be a problem, but I suppose we want to be able to measure time differences (eg: packet queue times) via subtraction.