use std::steady_clock for intervals

[mattklein123]

The leap second brought to my attention that we are incorrectly using system_clock for computing time intervals when we should be using steady_clock.

http://en.cppreference.com/w/cpp/chrono/steady_clock

[mattklein123]

@danielhochman can you work on this as another warm up task.

[danielhochman]

will do @mattklein123