mds: ScrubStack: initial skeleton

This commit introduces the ScrubStack, which is going to be the
data structure handling all of the scrubbing logic. For now it's
just the bare outlines of handling a CDentry stack and some function
declarations that let you see the outlines of its interfaces.

Signed-off-by: Greg Farnum <gfarnum@redhat.com>
wip: ScrubStack next_to_scrub implementation

Presently blocked by CDir scrub_info setup and dentry selection logic
needing to all be written.