Implement supporting the usage of the Datadog format for HTTP correlation

[mariusoe]

Currently, the agent only correlates incoming HTTP requests if the B3 header (trace-id, span-id, ...) are set.
In this ticket, the agent should be extend in order it is able to receive correlation headers of Datadog (X-Datadog-Trace-ID and X-Datadog-Parent-ID).
See also the following link how datadog generates their ids: https://github.com/DataDog/dd-trace-java/blob/master/dd-trace-api/src/main/java/datadog/trace/api/DDId.java

The agent should be configurable in order to put the previous mentioned correlation headers into outgoing HTTP requests.