EffectiveIndexInterval calculation is incorrect

I'm not familiar enough with the calculation itself to understand why this is happening, but see discussion on CASSANDRA-8851 for the background. I've introduced a test case to look for this during downsampling, but it seems to pass just fine, so it may be an artefact of upgrading.

The problem was, unfortunately, not manifesting directly because it would simply result in a failed lookup. This was only exposed when early opening used firstKeyBeyond, which does not use the effective interval, and provided the result to getPosition().

I propose a simple fix that ensures a bug here cannot break correctness. Perhaps thobbs can follow up with an investigation as to how it actually went wrong?