[Feature] Drop SQLite in favor of MongoDB

[ghost]

suggestion: remove SQLite footprint from ModelDB and use MongoDB as sole database.

use case: to have ModelDB running in production, we have it running in a Docker container with the Mongo database mounted as a volume from the host OS outside container. this allows us to quickly respawn in case the container goes down, and enables data recovery. with regards the SQL db, it's not only redundant given the introduction of the Mongo db, but its path is also hardcoded, so mounting it separately is more of a hack.