channeldb+invoices: add invoice htlcs

This commit adds a set of htlcs to the Invoice struct and
serializes/deserializes this set to/from disk. It is a preparation for
accurate invoice accounting across restarts of lnd.

A migration is added for the invoice htlcs.

In addition to these changes, separate final cltv delta and expiry
invoice fields are created and populated. Previously it was required
to decode this from the stored payment request. The reason to create
a combined commit is to prevent multiple migrations.

channeldb/db.go

@@ -110,6 +110,11 @@ var (
 			number:    10,
 			migration: migrateRouteSerialization,
 		},
+		{
+			// Add invoice htlc and cltv delta fields.
+			number:    11,
+			migration: migrateInvoices,
+		},
 	}
 
 	// Big endian is the preferred byte order, due to cursor scans over