Currently windows support was relying on Symlink as
a way to detect a drive, this doesn't work in latest
Windows 2016, fix this to use a proper mechanism by
using win32 APIs.
Additionally also add support for detecting bind mounts
on Linux.
disk usage crawling is not needed when a tenant
is not sharing the same disk for multiple other
tenants. This PR adds an optimization when we
see a setup uses entire disk, we simply rely on
statvfs() to give us total usage.
This PR also additionally adds low priority
scheduling for usage check routine, such that
other go-routines blocked will be automatically
unblocked and prioritized before usage.
