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.
new txs that it observes. The block manager alerts the fee estimator
of new and orphaned blocks.
Check for invalid state and recreate FeeEstimator if necessary.
