This commit consists of changes, which need to reside in a single
commit, so that the tests pass on each of the commits.
1. Remove do_make_flat_reader which disabled reverse reads by making the
slice a forward one. Remove call to get_ranges which would do
superfluous reversal of clustering ranges.
2. test: cql_query_test: remove expectation that the test_query_limit
fails for reversed queries, since reversed queries no longer require
linear memory wrt. the result size, when paginated.
771f3b12bd