Move common classes pertaining to databases from libkea-dhcpsrv to their own libraries

Configuration Backend structure described here: https://gitlab.isc.org/isc-projects/kea/wikis/designs/configuration-in-db-design#configuration-backend-structure calls for moving some of the existing classes from libkea-dhcpsrv to new libraries. For example: DatabaseConnection, DbLogger should go to libkea-database. This ticket covers such refactoring.

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