storage_proxy: fix check for singular range on read
storage_proxy: fix check for singular range on read

A singular range which a ring_position that does not contains a key may
operate on more than one key and thus is not really singular.

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