db: simplify mutation_partition::apply()
db: simplify mutation_partition::apply()

Since merging cells is a different operation for atomic cells and
collections, move it into compare_for_merge(), which is where we check
the column type.  Rename compare_for_merge to merge_column(), since it
now does more than compares.

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