Direct motivation for this is to be able to use two index readers from
a single mutation reader, one for lower bound of the range and one for
the upper bound of the range, without sacrificing optimization of
avoiding index reads when forwarding to partition ranges which are
close by. After the change, all index readers of given sstable will
share index buffers, so lower bound reader can reuse the page read by
the upper bound reader.
The reason for using two readers will be so that we are able to skip
inside the partition range, not only outside of it. This is not
possible if we use the same index reader to locate the upper bound of
the range, because we may only advance the cursor.
