Snapshotting API for State Stores

The parent ticket is KAFKA-3184.

The goal of this ticket is to provide a general snapshotting API for state stores in Streams (not only for in-memory but also for persistent stores), where the snapshot location can be either local disks or remote storage.

Design scope is primarily on:

1. the API design for both checkpointing as well as loading checkpoints into the local state stores
2. the mechanism of the checkpointing, e.g. whether it should be async? whether it should be executed on separate threads? etc.