Make internal timestamp unit configurable

[kjnilsson]

The timestamp that is captured is hard-coded to milliseconds which mean latency samples on very fast systems may be truncated to 0. Changing it directly to microseconds is a breaking change as the timestamp is passed to the ra_machine:apply/3 callback. Given this we could make it optionally overridable by providing an optional new ra_machine callback to get the current system time. E.g:

ra_machine:system_time()

We'd have to measure the potential performance impact of doing this for every command as well as every applied command.