Move the reception timestamping from the sinks to the REST handler

Implements the issue https://github.com/telefonicaid/fiware-connectors/issues/53

100% tests passed
Manual tests regarding HDFS and MySQL sinks
Given they come from HTTP headers and are not used for permistence purposes in sink/backends, I'd suggest to put CONTENT_TYPE and USER_AGENT in the `//HTTP header`section above.