Change nextUri slug for every request

[findepi]

Currently the slug is constant for the duration of the query. Usually, it is known only to the querying user, so it's not a problem.
However, it is logged to http-request.log. Everyone with access to the file can intercept query results.
Furthermore, when using log shipping, the http-request.log contents may be accessible to more people.

For increased security around query slug, each nextUri should be authorized with a different token.

[findepi]

nextUri slug can be HMAC(query slug, nextToken)