There was a race incorrectly counting transactions with one
confirmations to the unconfirmed balance instead of the confirmed
balance.
This happened because the balance cache of AddressSynchronizer got
invalidated after `on_event_blockchain_updated` and then again after
`receive_history_callback`->`add_transaction`, however when calling
`AddressSynchronizer.get_balance()` before the tx got spv verified the
height would still be counted as 0 (unconfirmed), populating the balance
cache again with the unconfirmed balance.
I noticed this only on QML due to timing differences to Qt.
Invalidating the cache in `AddressSynchronizer.add_verified_tx()` after
the tx got verified causes the balance to get recalculated and shown
correctly.
f321x
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