sstables: don't overwrite datafiles
sstables: don't overwrite datafiles

Prevent data loss resulting from erronously overwrting an sstable.

Reviewed-by: Raphael S. Carvalho <1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@cloudius-systems.com>

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