Avoid header duplication "x-rp-unique_id" and "x-rp-unique-id"

[oli-h]

Note the dash vs. the underscore in the header name.
The one with the underscore probably is a typo - but still both header names occur in various Gateleen source files - often with a fall-back implementation like here:
https://github.com/swisspush/gateleen/blob/v1.1.27/gateleen-routing/src/main/java/org/swisspush/gateleen/routing/StorageForwarder.java#L148-L151

Additionally, we have an header x-request-id which seems to have the same or very similar semantic.
There should be at least a proper documentation for both (or all three, if the underscore-version is intentional (and not a typo)

[mcweba]

I already started this task with the following Pull Request: #495.
We want to get rid of the x-rp-unique-id header and only use x-rp-unique_id and x-request-id. For the x-request-id header we should set this when no x-rp-unique-id is present in the request. Right now, x-request-id is only read, but newer written.