Optional replacement payload to dispatch instead of the raw wire
message — e.g. the anti-replay protector unwraps its signed envelope
(tag + clientId + clock + hmac + inner frame) down to the inner
y-protocol frame here. Without this, the envelope bytes themselves
would be handed to dispatchMessage, which doesn't recognise them as
a sync/awareness frame and silently drops them in its default case.
Optional replacement payload to dispatch instead of the raw wire message — e.g. the anti-replay protector unwraps its signed envelope (tag + clientId + clock + hmac + inner frame) down to the inner y-protocol frame here. Without this, the envelope bytes themselves would be handed to
dispatchMessage, which doesn't recognise them as a sync/awareness frame and silently drops them in itsdefaultcase.