Move stats_tracker.go into the extension API and increment MatchedPairs from inside the Emit method (#272)

* Move `stats_tracker.go` into the extension API and increment `MatchedPairs` from inside the `Emit` method

* Replace multiple `sync.Mutex`(es) with low-level atomic memory primitives

tap/api/api.go

@@ -87,6 +87,7 @@ type Dissector interface {
 }
 
 type Emitting struct {
+	AppStats      *AppStats
 	OutputChannel chan *OutputChannelItem
 }
 
@@ -96,6 +97,7 @@ type Emitter interface {
 
 func (e *Emitting) Emit(item *OutputChannelItem) {
 	e.OutputChannel <- item
+	e.AppStats.IncMatchedPairs()
 }
 
 type MizuEntry struct {