Add support for 'zookie' protocol as described in Zanzibar spec

[akajla09]

Is your feature request related to a problem? Please describe.
The Google Zanzibar paper describes a 'zookie' protocol that:

• allows clients to dictate desired 'freshness' for authz checks based on a supplied zookie token
• gives the server flexibility to implement caches that optimize for latency given clients' tolerance for 'freshness'

Describe the solution you'd like
Implement a similar 'zookie' protocol in the warrant server. Specifically:

• Server should pass back an opaque 'Warrant-Token' on API calls that clients can cache and pass on subsequent checks. This token bookmarks a specific write and allows the server to determine its own 'freshness' on later checks.
• At check time, the server compares the client-passed token to its own token(s) (could be multiple if multiple caches) and utilizes a cache/datastore that is 'at least as fresh as' the client-passed token to serve the request.
• If a client does not pass a Warrant-Token, use the most up-to-date store (likely the db) to serve the request (preserving current functionality for backwards-compatibility)