dht/boot_strapper: Use unordered_set for tokens
dht/boot_strapper: Use unordered_set for tokens

unordered_set is used everywhere for tokens. This makes it is easier to
construct a boot_strapper object in storage_service::bootstrap where
unordered_set is used for tokens.

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