[bz5441909] request-cycle events in mojito

[add0n]

It would be nice to have a way to attach listeners into some request-cycle events like end-headers, triggered just before the first byte of the content been sent.

The case I want to address here is to store all different settings we want to store on a cookie and send it as late as possible. So we can have different components managing their settings independently.

Please let me know if you need more details about it.