invoices: use lntypes.Hash and lntypes.Preimage

Previously chainhash.Hash was used, which converts to/from string in
reversed format. Payment hashes and preimages are supposed to be
non-reversed.

channeldb/invoices.go

@@ -10,6 +10,7 @@ import (
 
 	"github.com/btcsuite/btcd/wire"
 	"github.com/coreos/bbolt"
+	"github.com/lightningnetwork/lnd/lntypes"
 	"github.com/lightningnetwork/lnd/lnwire"
 )
 
@@ -105,7 +106,7 @@ type ContractTerm struct {
 	// PaymentPreimage is the preimage which is to be revealed in the
 	// occasion that an HTLC paying to the hash of this preimage is
 	// extended.
-	PaymentPreimage [32]byte
+	PaymentPreimage lntypes.Preimage
 
 	// Value is the expected amount of milli-satoshis to be paid to an HTLC
 	// which can be satisfied by the above preimage.