Fixing channel status support to follow spec.

Motivation:
I had misunderstood the specification and have now updated the channel
status to be inline with the latest spec:
https://github.com/martinthomson/drafts/blob/master/push-http2/draft-thomson-webpush-http2.xml
Adding response headers for monitor request as per spec.

Motivation:
The specification states that a monitor request also return the channel
weblink and expiration information.
This is mentioned in section 4 [1] which is about registering:
"The push server SHOULD also provide the "channel" link and expiration
   information in response to requests to the "monitor" resource."

This is not mentioned in monitoring section [2] though.

[1] https://tools.ietf.org/html/draft-thomson-webpush-http2-01#section-4
[2] https://tools.ietf.org/html/draft-thomson-webpush-http2-01#section-6

Result:
A monitor request will now return the channel weblink header and also
'cache-control' header
Adding support for 'Prefer: wait=0' header with monitor.

Motivation:
When a device request to monitor it can also request that the server
send out any non delivered messages straight away by sending the Prefer
header.
This is described in the 'Monitor and Receiving Push Message' section
[1] of the specification.

[1] https://tools.ietf.org/html/draft-thomson-webpush-http2-01#section-6