[keyserver] Roll back D11838

Summary: This solved ENG-8072 in production. It appears that performance was significantly affected by converting this column's type. We'll need to find another approach here.

Test Plan: I deployed this to production and confirmed that it solved ENG-8072

Reviewers: tomek, ginsu, will

Reviewed By: ginsu

Differential Revision: https://phab.comm.dev/D11976