fix(rln-relay): chunk event fetching

[rymnc]

Problem

With a large difference in the block numbers between which we have to fetch all the MemberRegistered events, there is a chance that the rpc provider would reject the requests made, simply because it is too large a range.

Impact

medium

To reproduce

• (cannot repro, since this issue will exist on mainnet)

Expected behavior

• Chunk block syncs (100 blocks per fetch)

Additional context

• Follow how nimbus did it here - https://github.com/status-im/nimbus-eth2/blob/55ae7d3b00fcb7029ced737eef8868690cbaa756/beacon_chain/eth1/eth1_monitor.nim#L1852

Thanks for the report: @weboko @richard-ramos (ref: waku-org/js-rln#57 (comment))