nway_merger: allow for comparators without default constructors
nway_merger: allow for comparators without default constructors

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