Make logs hash and not logs part of block hash preimage

[benesjan]

Motivation

There is an an issue with L2Block.getTxs(). It throws when it's called on a block which does not have attached logs and the reason is that the L2Tx type which is returned tries to compute block hash and logs are part of block hash preimage. Since we serve blocks without logs this is a common issue.

Solution

Refactor L2Block type such that it always contains encrypted and unencrypted logs hashes. Make these logs hashes part of the block hash preimage instead of the logs themselves.

[benesjan]

No longer necessary with #4286 because block hash is now a hash of header.