chainntnfs+sweep: add LookupInputMempoolSpend and use it in the

sweeper

This commit implements a new method, `LookupInputMempoolSpend` to do
lookups in the mempool. This method is useful in the case when we only
want to know whether an input is already been spent in the mempool by
the time we call.

chainntnfs/mocks.go

@@ -2,6 +2,7 @@ package chainntnfs
 
 import (
 	"github.com/btcsuite/btcd/wire"
+	"github.com/lightningnetwork/lnd/fn"
 	"github.com/stretchr/testify/mock"
 )
 
@@ -39,3 +40,13 @@ func (m *MockMempoolWatcher) CancelMempoolSpendEvent(
 
 	m.Called(sub)
 }
+
+// LookupInputMempoolSpend looks up the mempool to find a spending tx which
+// spends the given outpoint.
+func (m *MockMempoolWatcher) LookupInputMempoolSpend(
+	op wire.OutPoint) fn.Option[wire.MsgTx] {
+
+	args := m.Called(op)
+
+	return args.Get(0).(fn.Option[wire.MsgTx])
+}