[keyserver] Move update delete condition to a spec

Summary:
Add a field to a spec defining delete condition for a type.

Depends on D9657

Test Plan: Generate some updates, reintroduce old code, and compare new with old result using _.isEqual

Reviewers: kamil, inka, bartek

Reviewed By: kamil

Subscribers: ashoat, wyilio

Differential Revision: https://phab.comm.dev/D9658