Fixup "Continuation: Add a new Continuation class."

Make a few changes to enable more sub-customization, like
for the MDSContinuation I'm adding.

Signed-off-by: Greg Farnum <gfarnum@redhat.com>
Rebase: MDS: Add an MDSContinuation for ease of use

Unlike the regular Continuation, this one works in terms of an MDRequest
and has wrappers to provide Context callbacks that are either
internal MDS or IO appropriate.

Signed-off-by: Greg Farnum <gfarnum@redhat.com>
MDS: clean up internal MDRequests the standard way

All cleanup is now routed through respond_to_request(),
which invokes the internal_op_finish Context*, then does
mdcache->request_finish(). This is easier to reason about,
and indeed fixes a bug (I was not cleaning up locks
following flush). Use the MDSContinuation to facilitate
this in scrub's case.

Signed-off-by: Greg Farnum <gfarnum@redhat.com>