feat(rln): track number of messages sent per epoch

[adklempner]

Description

Proper generation of an RLN proof requires supplying the message id, which is based on how many messages were sent by this membership for the current epoch.

User Story

Proposed Solution / Feature Design

RLN instance should track number of messages sent per epoch:

• update epoch when it changes (and reset messages sent)
• update number of messages sent (should this wait for successful light push response?)

Optional: Diagram or Draft of Design

Notes