multi: Add a channel.db migration.

The new migration removes the sweeper-last-tx top level bucket
from the channel.db database.

channeldb/migration31/store.go

@@ -0,0 +1,9 @@
+package migration31
+
+var (
+	// lastTxBucketKey is the key that points to a bucket containing a
+	// single item storing the last published sweep tx.
+	//
+	// maps: lastTxKey -> serialized_tx
+	lastTxBucketKey = []byte("sweeper-last-tx")
+)