Added SCTPTransport.BufferedAmount

It simplifies tracking amount of unsent data for all data channels,
including ones in closing state. Especially useful for IoT apps
which use multiple data channels and have to monitor memory usage.

sctptransport.go

@@ -430,3 +430,14 @@ func (r *SCTPTransport) association() *sctp.Association {
 
 	return association
 }
+
+// BufferedAmount returns total amount (in bytes) of currently buffered user data.
+func (r *SCTPTransport) BufferedAmount() int {
+	r.lock.Lock()
+	defer r.lock.Unlock()
+	if r.sctpAssociation == nil {
+		return 0
+	}
+
+	return r.sctpAssociation.BufferedAmount()
+}