channeldb: MigrateOutpointIndex, store indexStatus in outpoint index

Adds an outpoint index that stores a tlv stream. Currently the stream
only contains the outpoint's indexStatus. This should cut down on
big bbolt transactions in several places throughout the codebase.

channeldb/migration20/log.go

@@ -0,0 +1,14 @@
+package migration20
+
+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
+}