Add support context propagation

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Motivation:
When using the transport, we always need to pass some context from the context to the sessionFactory or handler, but with the current implementation, the original ServerRequest is completely been ignored, it would be nice that we pass this information down.

we have some kind of options:

1. with ThreadLocal
2. With reactor's contextWrite and deferContext
3. with an additional parameter.

I think the 3 is better, which will not bound to any implementation.

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I'm doing some prototyping, will send a pr later.