Remove historic buckets from wallet database

The big win here is that fresh wallets no longer need to scan the full blockchain during unlock; they subscribe from modules.ConsensusChangeRecent. The wallet db should also shrink considerably; I would estimate that over 95% of its current size is just storing historic outputs. This will go a long way towards decreasing Sia's memory footprint.

The downside is that ProcessedInputs only have a valid Value field if they were funded by a wallet address. The API documentation has been updated to reflect this change. This doesn't break any known code because siac and Sia-UI already filter out any inputs that don't come from a wallet address. Worth noting that the Bitcoin wallet also only shows input values for wallet-controlled addresses. For users needing to inspect the value of every input, tools such as the explorer are available.