In this commit, we update all the btcutil imports to point to the new
sub-module.
In the same commit, we also modify the recently added `btcutil/go.mod`
file as we need to continue pointing to the _old_ version of btcd, until
we merge this PR and push a new tag.
Rollback takes a block hash rather than a BlockStamp.
Increase rounds in TestEstimateFeeRollback to test dropping txs that have been in the mempool too long.