Enhancement X-Ref-Modified

Depends on #165 

- Adds the X-Ref-Modified to block's GET end point
- X-Ref-Modified is the last modification time of the reference data for the block
  - stores the data as 'reftime' in the metadata back-end
  - reftime is the UTC Unix Epoch of modification time.
    - SQLite by default uses UTC
    - Cassandra has a Unix Timestamp function gets the time data from 'now()'; unclear if 'now()' is UTC or local to the server.
    - We explicitly get the time for MongoDB using Python's time functions to ensure it is UTC Epoch.
- Updates SQLite Test suite to use self.assert* instead of assert &lt;expression&gt;