src/lib/dhcpsrv/mysql_host_data_source.cc
    MySqlHostDataSourceImpl::getVersion() - new function to
    fetch version accessible to impl constructor

    MySqlHostDataSourceImpl() - added schema validation after
    connecting

    MySqlHostDataSource::getVersion() - new uses impl method

src/lib/dhcpsrv/pgsql_host_data_source.cc
    PgSqlHostDataSourceImpl()) - added schema validation after
    connecting

    PgSqlHostDataSourceImpl::getVersion() - no longer uses
    pre-prepared statement