Lock/unlock the database

[noseshimself]

Due to legal requirements and to avoid any kind of "vendor lock in" I intend to write my own backup tool only using the exposed API. To do so reliably I would like to have a way to keep the database in a consistent state while performing some long-running operation. Such a lock would better be auto-cancelled as soon as the connection drops or after a timeout if it is not released by the client requesting it.