Keep round's members indefinitely

[poszu]

motivation

Currently, the list of members of the previous round is not persisted otherwise than with the proof (and available to fetch via /proofs endpoint). Members are kept in a per-round leveldb database (each round has its own DB). The DB is removed after the round is finished.

As a prerequisite for #200, we should keep present and past rounds' members indefinitely.

design proposal

• centralize round's DB - use a single database shared by all rounds
• each round would have its unique DB key prefix, i.e round-0- to be able to query members for a given round.
• create a centralized service for round management (querying members and submitting challenges to the open round) - move the logic out of the poet worker service