fix: x-ref modified for mongo and sqlite

- the sqlite driver needed a way to decrement references of block when a
  file is deleted, by adding this functionality, the changes in
  x-ref-modified were also taken care of.

- the mongo driver now has functionality for changing 'x-ref-modified'
  time, both when the block is assigned to a file, and when a file gets
  deleted (while having blocks assigned to it)

- bugfix: mongo driver when getting an orphaned block from blockstorage.
  the fix here was to include the scenario, when the metadata_block_id
  was not present in metadata ( as is the case with orphaned blocks)
fix: create a shallow copy of dictionaries
Merge pull request #210 from TheSriram/x_ref_modified_bugfix

fix: x-ref modified for mongo and sqlite