channeldb: BigSize migration, store zero-conf, scid-alias bits

This introduces a BigSize migration that is used to expand the width
of the ChannelStatus and ChannelType fields. Three channel "types"
are added - ZeroConfBit, ScidAliasChanBit, and ScidAliasFeatureBit.
ScidAliasChanBit denotes that the scid-alias channel type was
negotiated for the channel. ScidAliasFeatureBit denotes that the
scid-alias feature bit was negotiated during the *lifetime* of the
channel. Several helper functions on the OpenChannel struct are
exposed to aid callers from different packages.

The RefreshShortChanID has been renamed to Refresh.

A new function BroadcastHeight is used to guard access to the
mutable FundingBroadcastHeight member. This prevents data races.

channeldb/migration29/migration.go

@@ -0,0 +1,72 @@
+package migration29
+
+import (
+	"bytes"
+
+	"github.com/btcsuite/btcd/wire"
+	"github.com/lightningnetwork/lnd/kvdb"
+)
+
+var (
+	// outpointBucket is the bucket that stores the set of outpoints we
+	// know about.
+	outpointBucket = []byte("outpoint-bucket")
+
+	// chanIDBucket is the bucket that stores the set of ChannelID's we
+	// know about.
+	chanIDBucket = []byte("chan-id-bucket")
+)
+
+// MigrateChanID populates the ChannelID index by using the set of outpoints
+// retrieved from the outpoint bucket.
+func MigrateChanID(tx kvdb.RwTx) error {
+	log.Info("Populating ChannelID index")
+
+	// First we'll retrieve the set of outpoints we know about.
+	ops, err := fetchOutPoints(tx)
+	if err != nil {
+		return err
+	}
+
+	return populateChanIDIndex(tx, ops)
+}
+
+// fetchOutPoints loops through the outpointBucket and returns each stored
+// outpoint.
+func fetchOutPoints(tx kvdb.RwTx) ([]*wire.OutPoint, error) {
+	var ops []*wire.OutPoint
+
+	bucket := tx.ReadBucket(outpointBucket)
+
+	err := bucket.ForEach(func(k, _ []byte) error {
+		var op wire.OutPoint
+		r := bytes.NewReader(k)
+		if err := readOutpoint(r, &op); err != nil {
+			return err
+		}
+
+		ops = append(ops, &op)
+		return nil
+	})
+	if err != nil {
+		return nil, err
+	}
+
+	return ops, nil
+}
+
+// populateChanIDIndex uses the set of retrieved outpoints and populates the
+// ChannelID index.
+func populateChanIDIndex(tx kvdb.RwTx, ops []*wire.OutPoint) error {
+	bucket := tx.ReadWriteBucket(chanIDBucket)
+
+	for _, op := range ops {
+		chanID := NewChanIDFromOutPoint(op)
+
+		if err := bucket.Put(chanID[:], []byte{}); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}