ceph_objectstore_tool: MemStore needs a CephContext

Pass g_ceph_context to ObjectStore::create() needed by MemStore

Fixes: #9661

Signed-off-by: David Zafman <dzafman@redhat.com>
MemStore: Need set/get_allow_sharded_objects() to work for ceph_objectstore_tool

Keep a bool sharded in memory and use for set/get_allow_sharded_objects()
Create a file "sharded" in the data dir when sharded is true during _save()
Check for file "sharded" during _load()

Fixes: #9661

Signed-off-by: David Zafman <dzafman@redhat.com>
Merge pull request #2639 from ceph/wip-9661

Fix 9661: ceph_objectstore_tool doesn't work with memstore

Reviewed-by: Samuel Just <sam.just@inktank.com>