refactor api mariasql usage: open new connection for each graphql request

[Schnitzel]

we're sharing the mariasql connection object within the api container, this is great for memory usage, but unfortunately mariasql itself has a queue. So even if we have multiple graphql requests going on, the requests to the database are done sequently.
see mscdex/node-mariasql#17 (comment)

All queries are placed in a local queue (due to the fact that the MySQL/MariaDB protocol is synchronous).

therefore I think we should initiate a new mariasql object for each graphql request, reuuse during the graphql request and then destroy it after the request has been parsed