ignore synthetic transactions for "next possible nonce" calculations

Stacks transactions done over Bitcoin are always stored with nonce = 0 on the API DB. This, however, can cause problems if it was the first executed transaction for an account because the "next possible nonce" reported by the nonces endpoint would be shown as `1`.

We should either make those synthetic transaction nonces be `null` or `-1` or we should ignore them from the next nonce calculation query.

A related idea is to add a `synthetic` boolean column to the transactions table to mark these transactions and avoid them during nonce calculations