[lib] Refactor the entries reducer to return an object

Summary:
Returning an array isn't convenient anymore because we're introducing a new return value - the operations.

Depends on D12148

https://linear.app/comm/issue/ENG-3486/move-entrystore-to-sqlite

Test Plan: Tried creating, editing and deleting an entry. Flow.

Reviewers: kamil, marcin

Reviewed By: marcin

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D12143