Store OPC-UA subscriptions in Postgres

[manuio]

Currently we are using a CSV file to store subscriptions. When the service restart this file is used in order to relaunch these subscriptions but CRUD operations are not updated and this can be a source of bugs.

We must replace this file by a DB and implement all CRUD.

We could then think about a system to fetch active subscriptions and stop or start them if needed.

[bbokun]

@manuio Maybe a better solution is not to use any additional database, but to use the things themselves, as the complete information for an OPC subscription is in the metadata of the thing