pulse: do all delay calculations when we need it

Just store the values for calculating the delay in the hot path. Then
compute the delay only when we need it.

src/modules/module-protocol-pulse/stream.h

@@ -64,7 +64,11 @@ struct stream {
 	uint64_t ticks_base;
 	uint64_t timestamp;
 	uint64_t idle_time;
-	int64_t delay;
+	struct {
+		uint64_t buffered;
+		int64_t delay;
+		struct spa_fraction rate;
+	} delay;
 
 	uint32_t last_quantum;
 	int64_t requested;