os/FileStore: force temp objects into _TEMP temp collection

os/FileStore: force temp objects into _TEMP temp collection

We've removed the temp collection concept from the ObjectStore interface,
but the FileStore HashIndex will mix objects with different pools together
by hash id, which breaks the ordering.

Compensate by forcing objects with 'temp' pool ids (anything < -1) into
a parallel temp collection.  Hide this detail isnide FileStore, below the
ObjectStore interface and above the HashIndex backend.

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