Assume partial notes will get a single completion log

[nventuro]

(related to #11627)

Because we need to keep track of pending partial notes, it'd be very nice if we could guarantee that these are only going to get a single completion log. Otherwise we need to keep track of them forever and continuosly search for new logs, or to somehow identify the correct completion (which seems quite hard given we typically don't know how a partial note will be completed). #14363 would provide some of the tools required to achieve this - the second part would be the (only) sender having logic that prevents ever doing multiple completion. This means that partial notes would typically be completed by non-account contracts.

Interestingly, if the combination of #14363 and proper contract logic prevent multi-completion, then we don't need to emit a note completion nullifier (saving DA), since a double-spend would be impossible.

[nventuro]

With #14379, the sender now has the tools to guarantee partial notes get a single completion log. It is possible for bad partial note creators to complete their notes multiple times however. We have two options here:

• leave it up to the sender to do the right thing and expect a single completion log for the note. If the partial note were to be completed multiple times we'd not find the later completions
• emit a nullifier marking the partial note as consumed. If the sender does not implement proper checks, this would fail loudly instead of us silently not finding the second log. This would be more costly in terms of DA, and it's not clear the situation improves - a bad sender still results in a very bad scenario