perf: Optimize detecting deleted conversation after a backup restore [WPB-6166]

[atomrc]

WPB-6166 [Web] [4hrs] Investigate possible memory leakages due to backup restore

Description

After a successful backup restore, we try to locally delete conversations that are deleted on backend.
Previously we didn't have a way to get those deleted conversation other than querying every single conversation and checking if the response is a 404 on backend.

Now we can optimize this and query all the conversations at once and check which ones are not_found.

Checklist

• PR has been self reviewed by the author;
• Hard-to-understand areas of the code have been commented;
• If it is a core feature, unit tests have been added;