MemStore: update for the new ObjectStore interface

68fdcfa1cc249af859400a2ce4590fefbb2f525b changed the ObjectStore
interface in the 'next' branch, which was merged into master by
e5a02c33e23e4fbdc7bf0f16a5bbff61f4e37186. Unfortunately the
Memstore (added via the master branch) was not corrected for this
interface change.

Signed-off-by: Greg Farnum <greg@inktank.com>
Reviewed-by: David Zafman <david.zafman@inktank.com>