this is a performance optimisation.
measurements using a large wallet with 11k txns:
syncing XPUB for the first time takes 10 seconds. leaving window open, and
syncing same XPUB again in new window takes 30 seconds. in third window,
it takes ~50 seconds. then ~70s. presumably scaling linearly.
this is due to the history_list.update_item call being CPU-heavy.
now all of them take 10 seconds.
Comment is no longer relevant. Also, actually it was incorrect.
Each txn is only downloaded once, though 'added' multiple times to the wallet.
The triggers are only sent out by the Synchronizer, once, when downloaded.
The actual reason for the inconsistency was that get_wallet_delta can only
give complete results once the wallet is synced.