[#2685] implement a thread-safe switch db context manager

[nzjess]

Proposal to address thread-safe db switching, as per: https://github.com/MongoEngine/mongoengine/issues/2685

Doesn't yet address thread-safe collection switching, but this could probably be considered in a similar way.

[bagerard]

This will need some tests (involving multiple databases and multiple threads)

[igatanasov]

Hey folks, Any plans this to be merged and integrated in the framework?