feat: HEAD /blocks/{block_id} endpoint

Depends on #132 

on a HEAD, the block is checked for existence in the metadata store,
after which its checked for existence in block storage.

- If present in metadata, and non-existent in block storage:
     502 Bad Gateway is returned along with an explanation.

- If present both in metadata and block storage.:
     200 OK is returned along with headers such as
     x-block-reference-count

- If not present in metadata:
     404 Not Found is returned
Perhaps we should add the the message stored by ConsistencyError in the response?

This would be helpful for clients that are logging the connections. Otherwise the client developer will have to have access to the Deuce Server logs to figure out what exactly the inconsistency is.