[0.2.1-deb] Syncing unstar action in JI results in database lock error

[eloquence]

Originally reported by @rmol in his QA report; I was able to reproduce immediately.

Steps to reproduce

1. Start the client and log in (run via LOGLEVEL=DEBUG securedrop-client in sd-app)
2. Load Tor Browser in sd-proxy and visit the Journalist Interface; log in there as well
3. Star an unstarred source in the client (I selected the most recent one)
4. Reload in the Journalist Interface. The source should be starred.
5. Unstar the source in the Journalist Interface
6. Wait for the client to sync

Expected behavior

Client picks up the unstarring

Actual behavior

Star never updates. database is locked error logged at DEBUG level. Log details.

After restart, the unstar action is picked up.

[eloquence]

(Marked as release blocker not because of the likelihood of this scenario but because of the severity of the failure mode.)