Fixing RebuildKeyspaceGraph to support empty cells.

The logic was failing when a SvrShard doesn't exist.
We need to instead use an empty SvrShard (based on the Shard)
with an empty TabletTypes list.

Since the Shard objects might have just been updated (and not
propagated to local copy of global topo server), put them in a cache
so we don't re-read them.