Investigate removing old Ethereum events from storage

[sug0]

To save storage space over time, it might be nice to remove processed Ethereum events from the current state at some block height $H$. Any Ethereum event whose processing is governed by nonces can potentially take advantage of such an optimization. To check if an Ethereum event has been seen (e.g. reached $\ge \frac{2}{3}$ voting power by stake), we can check if its nonce is lower than the nonce of the Ethereum event of the same kind that is registered in storage.

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Further optimizations

• After confirming validator set update Ethereum events, we can delete their proof from storage. EthBridgeQueries::valset_upd_seen should check the nonce in storage of the last processed valset upd event.
• A similar optimization can be employed to Bridge pool root signatures.

[sug0]

Deleting a value from storage still writes a diff of the previous value. No data is actually being deleted, at the RocksDB level. Therefore, we can't free up any space derived from seen Ethereum events.

Closing this issue

[sug0]

This might still be relevant, since keys that aren't deleted carry over a diff from the previous block height, which still contains some data. Please confirm this @yito88

[sug0]

Changed the priority because the Ethereum Bridge is not undergoing active development, currently. However, before its launch, this should definitely be implemented, as it could allow garbage collecting old tallies from storage, reducing the required space to operate the bridge in Namada.