column family: add estimate read, write
column family: add estimate read, write

This adds an estimated read and estimated write histogram to the column
family stats object.

acbellini/scylla,wildinto/scylla,phonkee/scylla,justintung/scylla,gwicke/scylla,senseb/scylla,victorbriz/scylla,respu/scylla,avikivity/scylla,duarten/scylla,aruanruan/scylla,tempbottle/scylla,rentongzhang/scylla,scylladb/scylla,stamhe/scylla,senseb/scylla,raphaelsc/scylla,rluta/scylla,tempbottle/scylla,scylladb/scylla,linearregression/scylla,aruanruan/scylla,dwdm/scylla,duarten/scylla,respu/scylla,capturePointer/scylla,raphaelsc/scylla,respu/scylla,rluta/scylla,rluta/scylla,raphaelsc/scylla,bowlofstew/scylla,kangkot/scylla,phonkee/scylla,glommer/scylla,aruanruan/scylla,kjniemi/scylla,eklitzke/scylla,rentongzhang/scylla,eklitzke/scylla,victorbriz/scylla,phonkee/scylla,asias/scylla,capturePointer/scylla,tempbottle/scylla,acbellini/scylla,victorbriz/scylla,stamhe/scylla,justintung/scylla,rentongzhang/scylla,guiquanz/scylla,justintung/scylla,scylladb/scylla,asias/scylla,bowlofstew/scylla,acbellini/scylla,linearregression/scylla,avikivity/scylla,guiquanz/scylla,bowlofstew/scylla,eklitzke/scylla,kjniemi/scylla,kangkot/scylla,duarten/scylla,gwicke/scylla,kangkot/scylla,glommer/scylla,scylladb/scylla,asias/scylla,wildinto/scylla,shaunstanislaus/scylla,senseb/scylla,shaunstanislaus/scylla,dwdm/scylla,gwicke/scylla,shaunstanislaus/scylla,linearregression/scylla,avikivity/scylla,glommer/scylla,stamhe/scylla,kjniemi/scylla,wildinto/scylla,dwdm/scylla,capturePointer/scylla,guiquanz/scylla