Sync stops after longer runtime

[rvolz]

When running version 0.9.0 in reconciliation mode, the synchronization of messages stops suddenly. No new messages will be imported from the network. This happened so far after 26 to 40 days of continuous uptime.

Indicators that this happened are:

• the error logs from sync.mjs (handleLevels) stop appearing in the logs (see image)
• the list endpoint produces no new messages over longer periods, it might hang

The only remedy in this case is a restart. However, after a restart of this kind, the app creates a high load for 1–2 hours (DB reorg?). During this time, the list endpoint of the API is mostly unresponsive.