Ensure `possible_next_nonce` reports correct value for sponsored txs

[zone117x]

There's a report of a possible bug when the API calculates nonces for addresses associated with sponsored transactions. The report indicates the issue may be related to when the mempool is queried for nonces. This needs to be investigated to determine if there is an actual bug. Then the fix is likely straightforward.

[smcclellan]

We haven't heard from the original filer of this ticket, so I'm moving this to lower priority & icebox.