MB-8386: Better system stats on Windows

This change replaces the aging Sigar library on Windows
with a WMI based implementation while keeping the port
functionality and interface identical. With WMI in place
now, future commits can tap into the extensive statistics
that the platform provides via WMI.

Change-Id: Ia19e3b1d0954116c4de3b9f6ee1069fadbcadc55