channeldb+migration27: patch balance fields for historical chan

This commit adds a new migration to patch the two balance fields,
`InitialLocalBalance` and `InitialRemoteBalance` for the historical
channels. Because they are not saved previously, for historical channels
prior to the revocation log PR, these fields will be empty.

channeldb/migration27/log.go

@@ -0,0 +1,14 @@
+package migration27
+
+import (
+	"github.com/btcsuite/btclog"
+)
+
+// log is a logger that is initialized as disabled.  This means the package will
+// not perform any logging by default until a logger is set.
+var log = btclog.Disabled
+
+// UseLogger uses a specified Logger to output package logging info.
+func UseLogger(logger btclog.Logger) {
+	log = logger
+}