Because the BestBlock method of ChainIO is not exposed through any
RPC we want to get rid of it so we can use the sweeper outside of
lnd too. Since the chain notifier now also delivers the current best
block we don't need the BestBlock method any more.
This commit is a step to split the lnwallet package. It puts the Input
interface and implementations in a separate package along with all their
dependencies from lnwallet.
This commit fixes a test flake caused by a race condition. Using the
real sweeper in the nursery test created too complex concurrent
behaviour to reliably assert on and made the tests difficult to
comprehend.
