ignore synthetic transactions for "next possible nonce" calculations

[rafaelcr]

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