[lib] Refactor keyserver reducer pt.1

Summary:
issue: https://linear.app/comm/issue/ENG-4729/refactor-reducers
In this diff I refactored the firt two ifs of this reducer. The rest of the logic will be refactored in next diffs

Test Plan: Dispatched setNewSessionActionType and resetUserStateActionType, chceked that the reducer works as expected for them

Reviewers: michal, kamil, ginsu

Reviewed By: michal

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9750