Query engine: disable or restrict built-in full-text engine (backport to 1.0)

With Oak 1.0.x, the built-in full-text engine is used if no other full-text index is available. This is different than in newer versions of Oak, where the built-in engine is disabled in that case by default.

I think it makes sense to backport this to Oak 1.0.x, so that all versions behave the same way.

This is to prevent out-of-memory problems if no full-text index is configured.