memtable: Add non-const partition_entry::key() variant
memtable: Add non-const partition_entry::key() variant

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