feat: delete block endpoint

Blocks can now be deleted, if they have no references. A status
code of 204 is then returned.
A Constraint Error is raised by the metadata driver if there
are references, and 412 is returned as status code.

Endpoint:
    DELETE /v1.0/vaults/{vault}/blocks/{blockid}
Most times, we don't need try-except blocks, we have wrapped cassandra.  Sqlite is not critical for our deployment.  

but It's good to wrap the deletion and un-registration actions for safety.  It'd be good for addition and queries.  

Just my 2C.