fixed a check which was broken by commit 9f02ad43aa9415429d3b95b9b4f3762be946b915
minor changes to the spec
definitions for the new messages
use/declare arc_resource_t properly ... it shouldn't be a pointer since it already hides a void *
introducing shardcache_get_multi() arc_lookup_multi() and arc_lookup_nofetch()

on the way of supporting GET_MULTI commands down to the storage level

(for now implementing the support at the cache level)