osd/SnapMapper: include poolid in snap index

osd/SnapMapper: include poolid in snap index

We want to sort starting with (pool, snapid, ...) so that we align with
the structure of the purged_snaps.  Simply flattening all snaps across
pools is less than ideal because the purge records are intervals (with the
snap in the key the last snap for the interval); flattening means we'd have
to look at many records (across pools) to conclude anything.  Putting
these in the form we really want them simplifies things going forward.

Signed-off-by: Sage Weil <sage@redhat.com>