When preparing a statement, do not parse the provided string if we already have the parsed statement cached

If there are many clients preparing the same statement (or the same client preparing it multiple times), there's no point parsing the statement each times. We already have it prepared, we should ship back the prior result.

I would like us separately to consider introducing some checks to ensure that we never have a hash collision (and error if we do, asking the user to salt their query string), but this change in no way increases the risk profile here, since all we did was overwrite the prior statement with the new one. This change means that clients referencing the old statement continue to function and the client registering the colliding statement will not execute the correct statement, but this is in no way worse than the reverse situation.