sweep: use chain notifier instead of chain IO for best block

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.

sweep/sweeper_test.go

@@ -130,9 +130,8 @@ func createSweeperTestContext(t *testing.T) *sweeperTestContext {
 			ctx.timeoutChan <- c
 			return c
 		},
-		Store:   store,
-		Signer:  &mockSigner{},
-		ChainIO: &mockChainIO{},
+		Store:  store,
+		Signer: &mockSigner{},
 		GenSweepScript: func() ([]byte, error) {
 			script := []byte{outputScriptCount}
 			outputScriptCount++