Store each epoch in a separate database

[qubicmio]

What

The archiver should store the data of each epoch in it's own database so that we can start implementing the pruning of epochs.

Scope

• Keep backwards compatible (except stated).

Acceptance criteria

• The data of each epoch is in one database.
• Epoch transition works with new database handling.
• We might need a shared database part with metadata (can be calculated at startup).
• The rpc methods still work as before (except the identities transactions query endpoint).
• Number of empty ticks in epoch does not have to be available anymore.