[keyserver] Simplify updates merging

Summary:
The first step of moving merging updates into specs. The fact that we're doing the same thing in all the ifs makes it possible to simplify the code significantly.

Depends on D9658

Test Plan: Tested in combination with the next diff. Generated some updates, reintroduced the old code, and compared merged updates.

Reviewers: kamil, inka, bartek

Reviewed By: kamil

Subscribers: ashoat, wyilio

Differential Revision: https://phab.comm.dev/D9659