Fix parameters to SPV FetchHeadersProgress notification.
Fix parameters to SPV FetchHeadersProgress notification.

The first parameter to this notification was named lastHeaderHeight
but the implementation was always calling the notification with the
count of newly-fetched headers.

decred/dcrwallet,decred/dcrwallet,jrick/btcwallet,jrick/dcrwallet,jrick/dcrwallet,jrick/btcwallet