Discover a way to move proof key calculation into background/uniform process

[MavenRain]

The current proof implementation requires a direct treatment of collection of all of the keys used to serve a particular endpoint for submission for construction of a proof. It would be desirable to make this process uniform and automated in some way. However, the naive solution of creating a wrapper type that interacts with the underlying merk store runs into issues.

References:
Wrapper type issues - https://coda.io/d/LedgerStore-Proof-Wrapper-Design_dNuzHODZzxm/LedgerStore-Proof-Wrapper-Design_suJTS

Proof implementation PR - #245

Proof spec PR - many-protocol/specification#89