Clean up KeyRangeIterator classes

5.0 patch PR -  https://github.com/apache/cassandra/pull/3194 - Fully green CI --  [-https://app.circleci.com/pipelines/github/ekaterinadimitrova2/cassandra?branch=19428-5.0-3-] Latest branches: https://github.com/ekaterinadimitrova2/cassandra/tree/19428-5.0-5 https://github.com/ekaterinadimitrova2/cassandra/pull/new/19428-trunk-2 I tested locally the patch on top of the trunk - running the new fuzz tests added, and they all passed, too. I can propagate and run full CI when we confirm the final patch.   Clean up KeyRangeIterator classes replace KeyRangeConcatIterator's PriorityQeueu with List remove KeyRangeIterator.current and simplify remove injected exception and tests - not relevant to the current implementation expand randomized testing inline getCurrent() -> peek(); rename getCount to getMaxKeys redefine skipTo contract to not return a value (which saves unnecessary work when skipTo is called multiple times in a row) calling hasNext in skipTo is a pessimization; if the iterator is in DONE state, then skipTo will see it and avoid further effort; if it is not, then we are computing a next value that we're just going to throw away Fix a bug in QueryController#getIndexResults error handling