streaming_histogram: implement sum
streaming_histogram: implement sum

This function is used to estimate number of points in interval
[-inf,b]. It will be useful for estimating droppable tombstone
ratio in a given sstable.

Signed-off-by: Raphael S. Carvalho <1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@scylladb.com>

scylladb/scylla,scylladb/scylla,duarten/scylla,duarten/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla