tracker: record announce/scrape stats in handlers

This deduplicates code tracking the stats code tracking announces and
scrapes for each protocol. The down side is that it isn't aware of any
failures writing responses, but it was already being called before the
write to a response for HTTP.

tracker/announce.go

@@ -70,6 +70,7 @@ func (tkr *Tracker) HandleAnnounce(ann *models.Announce, w Writer) (err error) {
 		stats.RecordEvent(stats.DeletedTorrent)
 	}
 
+	stats.RecordEvent(stats.Announce)
 	return w.WriteAnnounce(newAnnounceResponse(ann))
 }