Adds MonitorThread
Adds MonitorThread

This thread will run in the background during zfs performance tests, and
periodically print out information. It could be turned into a bunch of
tuneables:
-print min by min or total average
-print every X seconds
but I am currently hoping I can get it descriptive enough that people
will not need those tuneables.

datto/zfs-tests,stevenburgess/zfs-tests