read_indexes() will not work for a column family that minimum
index interval is different than sampling level or that sampling
level is lower than BASE_SAMPLING_LEVEl.
That's because the function was using sampling level to determine
the interval between indexes that are stored by index summary.
Instead, method from downsampling will be used to calculate the
effective interval based on both minimum_index_interval and
sampling_level parameters.
Fixes issue #474.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>