New MySQL and PostgreSQL packages

Copy of !77 (merged).

• ce401a4b New MySQL and PostgreSQL packages

• 634ca4ef Move libkea-*sql.so from common to their own packages

• c710cc5d Add ChangeLog entries

• https://jenkins.aws.isc.org/job/kea-manual/job/pkg/439/
• https://jenkins.aws.isc.org/job/kea-manual/job/check-pkgs/57/