(PDB-4808) Validate the read-db at startup; migrate all write dbs
(PDB-4808) Validate the read-db at startup; migrate all write dbs

Rework the datbase initialization process to validate the
[read-database] too, and to migrate all write dbs.  (Though migrations
are still disallowed when multiple databses are specified.)

puppetlabs/puppetdb,puppetlabs/puppetdb,puppetlabs/puppetdb,puppetlabs/puppetdb,puppetlabs/puppetdb