Improve resilience on power outages

[drolbr]

See this thread on community.openstreetmap.org .

There are apparently some scenarios where an Overpass database is rendered unusable when a power outage stopped the disks. Improve resilience with regard to proper atomics and more checks on recovery startups to make Overpass more resilient.

[drolbr]

The update mechanism has skipped one diff. This must not happen under no circumstances.

Also, there are zeros in the idx files where none should be. Check on startup for suspicious file content and use alternative files where available.