Metadata persistence in HDFS and MySQL

Partially implements the issue https://github.com/telefonicaid/fiware-connectors/issues/41

100% tests passed
Manual tests passed
New issue created: https://github.com/telefonicaid/fiware-connectors/issues/75

"Fixed" in https://github.com/telefonicaid/fiware-connectors/commit/db7547408cbdd6ecc35c07846f6225eeb3af02fa