[services] Backup - send log - add case for DB

Summary:
Depends on D4438

We want to handle a situation when we save the records in the database (it didn't work in the tests).
We should set the persistence method to the DB when receiving a "tiny" chunk - "tiny" means below LOG_DATA_SIZE_DATABASE_LIMIT that is 400KiB

Test Plan: full test plan in D4556, for now the service builds

Reviewers: tomek

Reviewed By: tomek

Subscribers: ashoat, tomek, adrian, atul, abosh

Differential Revision: https://phab.comm.dev/D4554