Use monotonic_time for Erlang

[jlouis]

This is really a minor nitpick, but system_time is under the influence of NTP and shouldn't be used as a difference clock source. Use erlang:monotonic_time(native) and then convert the difference to milli-seconds. This is time-warp safe.

It probably will not hit your benchmark in any way, but it is important to get right in larger production systems.