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
Fixed in https://github.com/telefonicaid/fiware-connectors/commit/7fcbbc10e50d63ffd9d473aba312d3952f64e828