chore(rln-relay): Gracefully handle chain forks

[rymnc]

Background

After #1266 has been solved, an issue arises where a peer

1. Listens to a non-canonical chain
2. Sees updates to the membership group
3. Updates local Merkle tree accordingly
4. The eth node sends over the canonical chain, which does not have the uncle chain's membership group updates

This results in the peer having a Merkle tree that cannot validate messages on the spam protected content topic anymore, since the validity of the message is tied to the root of the local Merkle tree.

Details

A graceful reconciliation mechanism which handles forks in the chain, and rolls back updates to the Merkle tree for a given set of blocks.

Acceptance criteria

• Track block numbers with associated insertions and deletions to the membership group
• Revert the changes that uncle blocks have introduced to the Merkle tree

cc: @staheri14 @s1fr0