multi: move payment related code into own package

This commit moves most of the code into its own package. It is
the smallest code move possible without moving import cycles and
keeping the changes to the code base as small as possible during
refactor.

channeldb/db.go

@@ -203,11 +203,13 @@ var (
 			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),
+			// This used to be create payment related top-level
+			// buckets, however this is now done by the payment
+			// package.
+			number: 15,
+			migration: func(tx kvdb.RwTx) error {
+				return nil
+			},
 		},
 		{
 			// Add our existing payments to the index bucket created
@@ -450,7 +452,6 @@ var dbTopLevelBuckets = [][]byte{
 	invoiceBucket,
 	payAddrIndexBucket,
 	setIDIndexBucket,
-	paymentsIndexBucket,
 	peersBucket,
 	nodeInfoBucket,
 	metaBucket,