EVM: returns proper transaction hash

We went for a transaction hash as an identifier because external tools did not complain about the ad-hoc hash. But there are some tools that checks the hash's validity, we need to return a proper hash.

Either we:

1. Compute the hash in the OCaml proxy server, it's doable, we have the hashing algorithm available, but we duplicate the code.
2. We wait for the EVM node so it share the transaction's hashing code.

The bug (and the fix) can be tested against https://github.com/jackturnbull/hashing-issue.