[services] Backup - Database Items - Update attachment methods

Summary:
Depends on D4133

Renaming appendAttachmentHolder to addAttachmentHolders for consistency and using tools::parseAttachmentHolders in both BackupItem and LogItem.

Test Plan: Services can still be built normally. For a full test plan of this functionality, please see D4135

Reviewers: palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, palys-swm, Adrian, atul, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4134