Write bestblock records in wallets:
* Every 20160 blocks synced, no matter what (before: none during IBD)
* Every 144 blocks after IBD (before: for every block, slow)
* When creating a new wallet
* At shutdown
This should result in far fewer spurious rescans.
* Bugfix: output the correct best block hash (during IBD, it can
differ from the actual current best block)
* Add height to output
* Add hash_serialized, which is a hash of the entire UTXO state.
Can be useful to compare two nodes.
* Add total_amount, the sum of all UTXOs' values.
The original test (checking whether the transaction occurs in the
txindex) is not usable anymore, as it will miss anything already
fully spent. However, as merkle transactions (and by extension,
wallet transactions) track which block they were last seen being
included in, we can use that to determine the need for
rebroadcasting.
Pieter Wuille