Zend\Paginator\Adapter\DbSelect alternative solution to count, with subselect

This is a generic solution that should work across all database platforms (I've tested against the Zend_Db-Examples repository schema, and it works on Sqlite, MySQL, and Postgres.)

Ideally, in the future, we will have a module per vendor of database so that we can have platform specific solutions that take advantage of platform specific features.  For example, MySQL could use SQL_CALC_FOUND_ROWS.