storage_proxy: fix digest_request_resolve completion reporting
storage_proxy: fix digest_request_resolve completion reporting

Digest resolver is broken in a way that prevents read completion to
be reported if data arrives after enough digests for cl were already
received. This happens because the code tried to save on a state and
used _cl_responses as an indicator that completion was reported already,
but this is incorrect since there can be enough responses for cl, but no
data yet. Fix by introducing special state to track completion reporting.

Fixes #331

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