1.6.0 db upgrade may leave dcrwallet unsyncable in rpc mode

[jrick]

While dcrwallet is not running, if its tip block is reorged out of dcrd's chain, dcrwallet will not recover after the 1.6.0 db upgrade which removed all v1 cfilters and fetches missing v2 cfilters, because the reorged block is not known by dcrd. This needs to recover automatically by rolling back the missing blocks.