Add support for HTTP header x-correlation-id

[KarstenSchnitter]

The request instrumentation creates a correlation id. This used to look into the HTTP headers x-CorrelationID and x-vcap-request-id. This new implementation additionally looks into x-correlation-id with priority between the two other headers.

The downside of this change is, that it might not be aligned with the Gorouter logs. The Gorouter might log either HTTP header x-CorrelationID and/or x-correlation-id. But it will always create an x-vcap-request-id which is seen by the application. This might create deviations in log parsing for correlation ids, when the newly supported header is used.