move all db related modules from mm2_main into mm2_db

[onur-ozkan]

There are 2 reasons for this:

1- We already have a crate (mm2_db) for the database layer.
2- We cannot use the modules from mm2_main - They are not truly shared across other crates. This forces us to apply awkward workarounds (e.g., using signals through mpsc channels) which is not ideal.