Use current height when generating claims on block_disconnected

Use current height when generating claims on block_disconnected

The `height` argument passed to `OnchainTxHandler::block_disconnected`
represents the height being disconnected, and not the current height.
Due to the incorrect assumption, we'd generate a claim with a locktime
in the future.

Ultimately, we shouldn't be generating claims within
`block_disconnected`. Rather, we should retry the claim at a later block
height, since the bitcoin blockchain does not ever roll back without
connecting a new block. Addressing this is left for future work.