This renames some variables and functions to make it clear that they
refer to partitions and not rows.
Old versions of sstablemetadata used to refer to a row histogram, but
current versions now mention a partition histogram instead.
This patch doesn't change the exposed API names.
Signed-off-by: Rafael Ávila de Espíndola <espindola@scylladb.com>
Message-Id: <20181229223311.4184-2-espindola@scylladb.com>
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