struct Delays

Sets the above three delays. The initial values are zero. These methods are virtual so that

subclasses can be notified when the value changes. It is not necessary to override the default

implementations.

Sometimes, changes to delay must be copied to the mix threads. This is supported with the

return value: an optional pair `(T, F)` contains a function (F) that must execute on the

corresponding thread (T).

This is a single method, rather than three methods, to reduce expensive state changes. For

example, SplitterNode's internal buffer size is `downstream_output_pipeline_delay +

downstream_input_pipeline_delay`. If we update those fields separately, we might see two

updates and allocate two new buffers when one new buffer would be sufficient.

REQUIRED: a field can be non-nullopt only if it's legal to call the corresponding getter.