db/legacy_schema_tables: Fix merge_keyspaces()
db/legacy_schema_tables: Fix merge_keyspaces()

Keys that are in "entries_only_on_right" need to be looked up from
"after". Fixes a regression introduced in commit 5418e32
("map_difference: Simplify difference value").

Signed-off-by: Pekka Enberg <add4fcd06328a394f0ad91feda7ee057316dc5ed@cloudius-systems.com>

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