routing+channeldb: migrate MC store to use minimal Route encoding

Add a new mcRoute type that houses the data about a route that MC
actually uses. Then add a migration (channeldb/migration32) that
migrates the existing store from its current serialisation to the new,
more minimal serialisation.

channeldb/migration32/migration.go

@@ -0,0 +1,53 @@
+package migration32
+
+import (
+	"bytes"
+	"fmt"
+
+	"github.com/lightningnetwork/lnd/kvdb"
+)
+
+// MigrateMCRouteSerialisation reads all the mission control store entries and
+// re-serializes them using a minimal route serialisation so that only the parts
+// of the route that are actually required for mission control are persisted.
+func MigrateMCRouteSerialisation(tx kvdb.RwTx) error {
+	log.Infof("Migrating Mission Control store to use a more minimal " +
+		"encoding for routes")
+
+	resultBucket := tx.ReadWriteBucket(resultsKey)
+
+	// If the results bucket does not exist then there are no entries in
+	// the mission control store yet and so there is nothing to migrate.
+	if resultBucket == nil {
+		return nil
+	}
+
+	// For each entry, read it into memory using the old encoding. Then,
+	// extract the more minimal route, re-encode and persist the entry.
+	return resultBucket.ForEach(func(k, v []byte) error {
+		// Read the entry using the old encoding.
+		resultOld, err := deserializeOldResult(k, v)
+		if err != nil {
+			return err
+		}
+
+		// Convert to the new payment result format with the minimal
+		// route.
+		resultNew := convertPaymentResult(resultOld)
+
+		// Serialise the new payment result using the new encoding.
+		key, resultNewBytes, err := serializeNewResult(resultNew)
+		if err != nil {
+			return err
+		}
+
+		// Make sure that the derived key is the same.
+		if !bytes.Equal(key, k) {
+			return fmt.Errorf("new payment result key (%v) is "+
+				"not the same as the old key (%v)", key, k)
+		}
+
+		// Finally, overwrite the previous value with the new encoding.
+		return resultBucket.Put(k, resultNewBytes)
+	})
+}