Task/iotcore 122 frn composition

Generate FRN from the information in the headers.
While for organization (I understand that derived form `Fiware-Service` header) there is not problem, take into account that:

* A single request could include more than one path (see `Madrid/Gardens/ParqueNorte, Madrid/Gardens/ParqueOeste`example at https://forge.fi-ware.org/plugins/mediawiki/wiki/fiware/index.php/Publish/Subscribe_Broker_-_Orion_Context_Broker_-_User_and_Programmers_Guide#Entity_service_paths)
* A single request could involve more than one entity (e.g. an updateContext on E1 and E2 at the same time).

If the current approach is not able to cope with multi-path and multi-entity and that is not fixable in this PR (or it is not clear how to adress the issue) I'd add at least a couple of FIXME comment stating the limitations of the current approach.