EVM Node: Keep the result of `eth_blockNumber` in memory

(I know I said I wouldn’t open a MR today. I know)

Compared to the sequencer and observer modes, the RPC mode does not keep the head of the chain in memory, and instead rely the store and the Irmin context extensively. The main reason for this was that SQLite was reasonably fast, and that Irmin keeps a cache that we don’t clear (because we don’t commit).

That being said, as part of the Infra V2 project, we need to enable the eth_blockNumber RPC for the relay.network.etherlink.com enpdoint. We really want this endpoint to be as light as possible, and if we can make eth_blockNumber almost free, so be it.

It is a good thing we can!

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
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• Provide automatic testing (see the testing guide).
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