feat: delete blockstorage endpoint

- Allows to delete a block directly from block storage, if it exists and
  zero references. returns a HTTP 204

- If the block has references, returns HTTP 409

- If the block doesnt exist we return a 404

Bugfix Cassandra driver:

Changed up timestamp for reftime to be actually UTC, since Cassandra's
timestamp is a local timestamp along with a UTC offset like: -0400
you are right.  

Thanks!