[backup] Small refactor
Summary:
Small refactor in preparation for the log item. There will be 3 impl blocks: for general code, backup (compaction) handlers and log handlers. Also moved get_item_key as a static method of BackupItem.
Depends on D10406
Test Plan: cargo check
Reviewers: bartek, kamil
Reviewed By: bartek
Subscribers: ashoat, tomek
Differential Revision: https://phab.comm.dev/D10428