1.8.0 nextAddress function called twice with each new connected block generating unnecessary addresses

[dezryth]

With dcrwallet 1.8.0, if the wallet is left unlocked with ticketbuyer enabled, it looks like the function nextAddress is being called twice each time dcrwallet connects to a new block.

This was not an issue before 1.8.0. This bug will create a big gap of unused addresses. It also means users can't leave dcrwallet running with ticketbuyer enabled without this bug creating a bunch of unnecessary unused addresses which could make it necessary to use the gaplimit flag when restoring from seed in the future.