Add MD5 ETag as response header

Estuary calculates an Md5 checksum of the received contents and compares it with the ETag field of the http headers.

We can just disable that. But for security reasons it might make sense to keep it and add the checksum as a metadata to phobos (which could be used to check if the database has been corrupted) and send it as the ETag field of the request.

Keep in mind, that issue #81 (closed) removes the content sha from the calculated signature. The body is therefore not checked anywhere anymore and might be vulnerable. If this is a concern...

Has been decided! We add the checksum. Don't forget to enable the check in estuary again