channeldb/migration21: add body and tests for migration 21

In this commit, we migrate all wire messages in the database from the
`legacy` to the `current` encoding.

This affects the way we write the `CommitDiff` and `LogUpdates` struct
to disk. We also need to migrate the network results bucket in the
switch as it includes a wire message without a length prefix.

channeldb/db.go

@@ -18,6 +18,7 @@ import (
 	"github.com/lightningnetwork/lnd/channeldb/migration13"
 	"github.com/lightningnetwork/lnd/channeldb/migration16"
 	"github.com/lightningnetwork/lnd/channeldb/migration20"
+	"github.com/lightningnetwork/lnd/channeldb/migration21"
 	"github.com/lightningnetwork/lnd/channeldb/migration_01_to_11"
 	"github.com/lightningnetwork/lnd/clock"
 	"github.com/lightningnetwork/lnd/lnwire"
@@ -182,6 +183,12 @@ var (
 			number:    20,
 			migration: migration20.MigrateOutpointIndex,
 		},
+		{
+			// Migrate to length prefixed wire messages everywhere
+			// in the database.
+			number:    21,
+			migration: migration21.MigrateDatabaseWireMessages,
+		},
 	}
 
 	// Big endian is the preferred byte order, due to cursor scans over