fix: get orphaned blocks

- make sure that 'metadata_block_id' is not None, before making any
  queries to the metadata databases. In the case, its None, it means
  the Block is orphaned in storage, set both x-ref-modified and
  x-ref-count to be 0
Merge pull request #222 from TheSriram/cassandra_orphanedblock_bugfix

fix: get orphaned blocks