[lib] refactor keyserver register in keyserver-reducer to ops

Summary:
Convert logic to ops approach.

Creating separate diff because I am a bit confused - @inka is modifying the state instead of returning on purpose?

Test Plan: Register new user on native and make sure state is properly set

Reviewers: inka

Reviewed By: inka

Subscribers: ashoat, tomek, inka

Differential Revision: https://phab.comm.dev/D10767