atomic_cell: Add is_live() version which takes a tombstone
atomic_cell: Add is_live() version which takes a tombstone

It determines if the cell is live given its cointainer has this
tombstone.

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