db: don't use stringstream for collection mutation serialization
db: don't use stringstream for collection mutation serialization

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