add caching_options
add caching_options

This is basically a glorified map<sstring, sstring>, that does some validation
on the options. Analoguous structures in the past were put directly at
schema.hh, but I will keep this one separate because it got slightly more
complex than the average.

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

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