objectcache: Restore 'keyspace' for LocalServerCache service
Follows-up 746d67f5fcc5 which implicitly caused the APCUBagOStuff
object to no longer have a wiki-dependent keyspace. This meant
that all cache keys were shared across wikis, even if they used
makeKey() instead of makeGlobalKey() because the keyspace was
not defined (e.g. it was the string "local" for all wikis, instead
of a string like "enwiki").