storage_proxy: use parallel_for_each in mutate() instead of semaphore
storage_proxy: use parallel_for_each in mutate() instead of semaphore

If several mutation in a batch throw exceptions have_cl.broken() will be
called more then once. Fix this by dropping ad hoc have_cl and use
parallel_for_each() that does the same thing that current code is doing.

Fixes #297

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