Adding a method that allows us to execute the count query on a pager before the query is executed.

Summary: used for the new publisher API paging stuff.  All expected Doctrine tests still pass

Test Plan: ran doctrine tests, the ones that passed before the change still pass.  Still need to run application tests.

Reviewers: apringle, dcorkill

Reviewed By: apringle

Differential Revision: http://phab.diablomedia.corp/D393