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
bugfix: DO NOT delete blocks with zero references

i.e x-reference-count of 0, but still exisiting in metadata.
Merge remote-tracking branch 'upstream/master' into delete_blockstorage_endpoint

Conflicts:
	deuce/model/blockstorage.py
	deuce/tests/test_blockstorage.py
pep8: fix
Merge pull request #189 from TheSriram/delete_blockstorage_endpoint

feat: delete blockstorage endpoint