Assess and memoize (if appropriate) SignedTransaction::sha256_of_proto_encoding

[Fraser999]

We should run an ad hoc test under semi-realistic conditions which counts how many times SignedTransaction::sha256_of_proto_encoding is a repeated call on a SignedTransaction instance. In other words, if we only ever call this once per instance, the total will be zero. If we call it exactly thrice per instance, the total will be twice the number of instances.

Once we have this information, we can decide whether to implement memoization of the tx hash as a field in the tx, or leave as is.

After brief discussion with @SuperFluffy, it might be better to just make this hash a normal member field rather than a lazily-initialized OnceCell for example, since the hash is probably almost always calculated for every signed transaction. Confer before implementing any changes to agree on an approach.

┆Issue Number: ENG-697

[joroshiba]

This issue is stale because it has been open 45 days with no activity. Remove stale label or this issue
be closed in 7 days.

[SuperFluffy]

We should just hash the signed transaction upon construction and not worry about memoization - I can't think of a scenario where we would construct a signed transaction without immediately logging its hash in some form.