sstables: Defer full checksum calculation
sstables: Defer full checksum calculation

Optimize checksum calculation by deferring "full checksum" update until
we've computed a full per-chunk checksum.

Signed-off-by: Pekka Enberg <add4fcd06328a394f0ad91feda7ee057316dc5ed@cloudius-systems.com>

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