Fixed hard tabs in monitor.cpp

From: Niklas Nielsen <nik@qni.dk>
Review: https://reviews.apache.org/r/15500
Fixed string leaks in os.hpp.

From: Niklas Nielsen <nik@qni.dk>
Review: https://reviews.apache.org/r/15368
Added list of TaskStatuses to Task message.

Task now includes repeated task status with timestamps.  This permits
caltulating the start/finish time for tasks (among other things).

To support this, a timestamp was added to the TaskStatus message which
is also passed to frameworks when there are state transitions.

Review: https://reviews.apache.org/r/14434
Normalized task start/finish time in Web UI.

Review: https://reviews.apache.org/r/15539