Investigate a way to work around Gerrit's resource lock issues

While it's unclear why exactly the lock occurs, it seems that either Gerrit does not have an action queue, or the REST API somehow bypasses such internal checks. Either way, on occasion, POST/PUT requests sent by the cherry-pick bot via the REST API sometimes receive back the error `409: com.google.gerrit.git.LockFailureException: Update aborted with one or more lock failures: PackedBatchRefUpdate[ ...`