mapper: remove Batcher interface, rename to Batcher struct

Remove the Batcher interface since there is only one implementation.
Rename LockFreeBatcher to Batcher and merge batcher_lockfree.go into
batcher.go.

Drop type assertions in debug.go now that mapBatcher is a concrete
*mapper.Batcher pointer.

hscontrol/mapper/mapper.go

@@ -44,7 +44,7 @@ type mapper struct {
 	// Configuration
 	state   *state.State
 	cfg     *types.Config
-	batcher Batcher
+	batcher *Batcher
 
 	created time.Time
 }