While we're iterating over the fetched keyspace names, some of these
keyspaces may get dropped. Handle that by checking if the keyspace still
exists.
Also, when retrieving the replication strategy from the keyspace, store
the pointer (which is an `lw_shared_ptr`) to the strategy to keep it
alive, in case the keyspace that was holding it gets dropped.
Closes#10861
ff4ecfa182