Enhancement X-Ref-Modified

- 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;