mutation: add a move assignment operator
mutation: add a move assignment operator

For that to work, we need to unfortunately lose the const specifier in the decorated_key

Signed-off-by: Glauber Costa <f8b79557d19b40666ebe336545a0b6178af65685@cloudius-systems.com>

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