Mirroring replica::database::get_keyspaces(), for Thrift's use.
We return a vector instead of a hash map. Random access is already
available via database::find_keyspace(). The name is available
via the keyspace metadata, and in fact Thrift ignore the map
name and uses the metadata name. Using a simpler type reduces
include dependencies for this heavily used module.
The function is plumbed to replica::database::get_keyspaces() so
it returns the same data.
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