fix(router): handle stateless cancel
A stateless CANCEL may occur when the caller sends it after the callee has already sent 200 and the proxy has destroyed the ForkCtx. In this case, the proxy must act as a stateless proxy and forward the CANCEL.
Also, stop processing of event when the ForkCtx exists. This avoid a 2nd CANCEL request creation in module forward.
Edited by Émilie Gerbaud