blockchain.read_header is expensive. do cheap tests first
on a wallet with 11k txns, that is synced except for spv proofs,
finishing sync now takes 80 sec instead of 180 sec
related: 41e088693d
If our guess of a txn getting confirmed at the same height in the new chain
as it was at in the old chain is incorrect, there is a race between the
verifier and the synchronizer. If the verifier wins, the exception will cause
us to disconnect.
fixes race between verifier and block header download.
scenario: client starts, connects to server. while client was offline,
there was a reorg. txn A was not mined in the old chain, but is mined
after reorg. client subscribes to addresses and starts downloading headers,
concurrently. server tells client txn A is mined at height H >= reorg height.
client sees it has block header at height H, asks for SPV proof for txn A.
but the header the client has is still the old one, the verifier was faster
than the block header download (race...). client receives proof. proof is
incorrect for old header. client disconnects.
