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/app.go

@@ -101,7 +101,7 @@ type Headscale struct {
 	// Things that generate changes
 	extraRecordMan *dns.ExtraRecordsMan
 	authProvider   AuthProvider
-	mapBatcher     mapper.Batcher
+	mapBatcher     *mapper.Batcher
 
 	clientStreamsOpen sync.WaitGroup
 }