watchtower/wtdb: add migration code for AckedUpdates

In this commit, the code for migration 4 is added. This migration takes
all the existing session acked updates and migrates them to be stored in
the RangeIndex form instead. Note that this migration is not activated
in this commit. This is done in a follow up commit in order to keep this
one smaller.

watchtower/wtdb/migration4/log.go

@@ -0,0 +1,14 @@
+package migration4
+
+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
+}