Refactor refreshing ztx confirmations

[leto]

The current code for refreshing confirmation counts for ztx's is extremely inefficient, it does a gettransaction call on every single ztx in the history of the wallet, to update the current confirmations count. No user actually cares about the conf count, except to know if it's unconfirmed, confirmed or notarized.

So we are spending a massive amount of CPU every few seconds, slowing down the GUI, in wallets with many transactions.

A better GUI would show simply unconfirmed/confirmed/notarized, and only when viewing individual transaction details would we show exact confirmation counts. In my experience, users do not care about the confirmation counts of old transactions at all. We should not slow down our GUI rendering them all the time.