Metadata persistence in HDFS and MySQL

Partially implements the issue https://github.com/telefonicaid/fiware-connectors/issues/41

100% tests passed
Manual tests passed
Fixed in https://github.com/telefonicaid/fiware-connectors/commit/fa48bfbce9a4882253ea19b43c9b85b3be734324 (impl), https://github.com/telefonicaid/fiware-connectors/commit/470239cd836fe8d7a5981923b76584454fff2260 (doc) and https://github.com/telefonicaid/fiware-connectors/commit/767f601f26ff57679720bd4ddb58f0927582611c (tests)