Prototyping end to end workflow and UpstreamFilter and Tests
Add session filter and backend

This adds session primitives and tests. This also has mock in-memory
store implementations of data stores for Secrets, Sessions, and Tokens.
It also includes JSON encoding/decoding for Tokens.

Fixes some build and test dependencies.
Merge pull request #5 from trane/session_and_proto

Merge Session backend and filter/service prototypes