Add a new next_replication column to system_schema.keyspaces table.
While there is an ongoing RF change:
- next_replication keeps the target RF values;
- existing replication_v2 column keeps initial RF values - the ones we
started the RF change with.
DESCRIBE KEYSPACE statement shows replication_v2.
When there is no ongoing RF change for this keyspace, its
next_replication is empty.
In this commit no data is kept in the new column.
72bb3113ac