context: scale max_quantum and add quantum_limit

Also scale the max_quantum with the selected rate. Add a new
quantum_limit property that is the upper limit of the quantum regardless
of the sample rate, this is usually the allocated buffer size.

See #1931

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

@@ -52,6 +52,7 @@ struct defs {
 	struct sample_spec sample_spec;
 	struct channel_map channel_map;
 	uint32_t max_quantum;
+	uint32_t quantum_limit;
 };
 
 struct stats {