Support per-class sub-object limit/offset

Support per-class sub-object limit/offset

Change the signature for in-db unapi functions so that the limit /
offset arguments become HSTORE values, rather than plain INTEGER values.
This gives you the ability to specify limits and offsets at both the
volume and copy level, via the likes of 'acp=>5, acn=>10' to
specify 5 copies and 10 volumes.

Signed-off-by: Mike Rylander <mrylander@gmail.com>
Signed-off-by: Dan Scott <dan@coffeecode.net>
Signed-off-by: Jason Stephenson <jstephenson@mvlc.org>