Fix ambiguous column names in Maniphest blocking queries

Summary: Fixes T7421. Now that we join the task table again to ignore closed blockers, all the column names are ambiguous. Make them unambiguous.

Test Plan: Issued some searches with various different parameters.

Reviewers: btrahan, joshuaspence, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T7421

Differential Revision: https://secure.phabricator.com/D11922