[keyserver] move logic to fetch not acknowledged policies to separate function

Summary: This is just a refactor, in next diffs I'll introduce changes that will require checking for not acknowledged policies and this will make possible to avoid try {} catch () {} pattern by using simple function call and improve readability.

Test Plan: Maintain previous functionality of this function

Reviewers: inka, michal, przemek

Reviewed By: inka

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6091