Spec'ing state saving and loading

[skrawcz]

Use cases:

1. User controls how the app is started via with_state() and with_entrypoint().
2. User delegates to a persister & burr to reload state for a given partition key, app_id -- via with_db() and with_identifier. Seqeuence id is either 0 or loaded from state, or passed in via with_identifier.
3. user can add a persister via with_persistence -- need to deduplicate hooks potentially

Notes:

• tracker & persister can be same object
• can have separate persister and tracker

Requirements:

• app knows about partition key
• app knows about app_id or generates one
• app sets sequence id to 0 unless it loads one from prior state
• hooks get passed partition key, app_id, sequence no, position

[elijahbenizzy]

Implemented by #73