[services] Lib - Add Base Database Manager

Summary:
Depends on D3951

There is some mutual code in DatabaseManagers across services. I think we can create a parent class that includes it.

Test Plan:

cd services
yarn run-local-cloud
yarn test-backup-service-dev-mode
yarn test-blob-service-dev-mode

Reviewers: palys-swm, ashoat

Reviewed By: ashoat

Subscribers: ashoat, palys-swm, Adrian, atul, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3956