channeldb: index payments by sequence number

Add an entry to a payments index bucket which maps sequence number
to payment hash when we initiate payments. This allows for more
efficient paginated queries. We create the top level bucket in its
own migration so that we do not need to create it on the fly.

When we retry payments and provide them with a new sequence number, we
delete the index for their existing payment so that we do not have an
index that points to a non-existent payment.

If we delete a payment, we also delete its index entry. This prevents
us from looking up entries from indexes to payments that do not exist.

channeldb/db.go

@@ -16,6 +16,7 @@ import (
 	mig "github.com/lightningnetwork/lnd/channeldb/migration"
 	"github.com/lightningnetwork/lnd/channeldb/migration12"
 	"github.com/lightningnetwork/lnd/channeldb/migration13"
+	"github.com/lightningnetwork/lnd/channeldb/migration16"
 	"github.com/lightningnetwork/lnd/channeldb/migration_01_to_11"
 	"github.com/lightningnetwork/lnd/clock"
 	"github.com/lightningnetwork/lnd/lnwire"
@@ -144,6 +145,19 @@ var (
 			number:    14,
 			migration: mig.CreateTLB(payAddrIndexBucket),
 		},
+		{
+			// Initialize payment index bucket which will be used
+			// to index payments by sequence number. This index will
+			// be used to allow more efficient ListPayments queries.
+			number:    15,
+			migration: mig.CreateTLB(paymentsIndexBucket),
+		},
+		{
+			// Add our existing payments to the index bucket created
+			// in migration 15.
+			number:    16,
+			migration: migration16.MigrateSequenceIndex,
+		},
 	}
 
 	// Big endian is the preferred byte order, due to cursor scans over
@@ -257,6 +271,7 @@ var topLevelBuckets = [][]byte{
 	fwdPackagesKey,
 	invoiceBucket,
 	payAddrIndexBucket,
+	paymentsIndexBucket,
 	nodeInfoBucket,
 	nodeBucket,
 	edgeBucket,