In the following commits, we reduce the number of the
migration_manager::anounce calls by merging some of them in a way
that logically makes sense. Some of these merges are similar --
we announce a new keyspace and its tables together. However,
we cannot use the current prepare_new_column_family_announcement
there because it assumes that the keyspace has already been created
(when it loads the keyspace from the database). Luckily, this
assumption is not necessary as this function only needs
keyspace_metadata. Instead of loading it from the database, we can
pass it as a parameter.