[DO NOT MERGE]: V3 Proof of Concept – SessionStore

[sshropshire]

Summary of changes

• Initial Concept Implementation of a SessionStore
• We can use session stores to encapsulate existing state and decouple internal state from feature clients
• We can give merchants access to this state as an opaque string to allow recovery from process kill

Checklist

• [] Added a changelog entry

Authors

List GitHub usernames for everyone who contributed to this pull request.

• @sshropshire