channeldb/migration33: migrate MC store pairs to default namespace

In this commit, the mission control store is migrated such that all
existing pairs which are currently stored directly in the top level
results bucket are now instead moved to a "default" namespace bucket.

Note that this migration is not yet invoked in this commit. The
migration will be invoked in the same commit that starts writing and
reading the new format.

channeldb/migration33/log.go

@@ -0,0 +1,14 @@
+package migration33
+
+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
+}