This commit introduces a new implementation of `create_single_key_sstable_reader`
in `time_series_sstable_set` dedicated for TWCS-created sstables.
It uses the fact that such sstables are mostly disjoint with respect to
contained `position_in_partition`s in order to decrease the number of
sstable readers that are opened at the same time.
The implementation uses `clustering_order_reader_merger` under the hood.
The reader assumes that the schema does not have static columns and none
of the queried sstable contain partition tombstones; also, it assumes
that the sstables have the min/max clustering key metadata in order for
the implementation to be efficient. Thus, if we detect that some of
these assumptions aren't true, we fall back to the old implementation.
f0842ba34e