Metadata persistence in HDFS and MySQL

Partially implements the issue https://github.com/telefonicaid/fiware-connectors/issues/41

100% tests passed
Manual tests passed
Yes, it seems complex and out of the scope this PR... let's move this to an issue in github (I'd suggest to re-use your previous comment in the issue body, as it describes pretty well the problem and a possible solution).

Regarding this PR, a FIXME with a reference (URL) of the issue will be enough in the code.