Use `NoteId` instead of `Word` in the mempool

[Mirko-von-Leipzig]

Not related to this PR, but was there any reason why note identifiers within the mempool graph were typed as Word? Maybe because we used what used to be known as note commitment back then which didn't have a newtype. Would be nice for it to be NoteId now

Originally posted by @igamigo in #2563 (comment)