[chain] Add parallel execution for transactions

[patrick-ogrady]

We should make sure that we don't include the program code key in the conflict set, otherwise we will only be able to execute one transaction per contract at-a-time (avoids the benefits of the already implemented state key design that allows for concurrent reads/writes from the same program).

This may have implications on how programs can be updated (as that would require including the code key in the conflict set as it could be modified).