[5.x] Fix search index race condition

[jasonvarga]

This PR solves a race condition when reading and writing a Comb-driven search index.

It's possible that one request will try to read the JSON file when another is writing to it, causing the JSON parsing to return invalid JSON.

Invalid JSON would return null. Passing null to collect() would be an empty collection. If this happened when you are trying to insert something into the index, it would result in that index only containing the inserted data.

This is solved by throwing an exception when encountering invalid JSON. This would prevent the search index being emptied out but of course now results in an exception.

Additionally, file locks are now used when reading and writing those files to prevent getting into that situation where the exception would be thrown to begin with.