In the following changes, we allow to alter from numeric rf to rack list.
Before the alter, two tablets of the same keyspace can have replicas
on different racks. To switch to rack list, we need to co-locate
the replicas. It will be achieved by pausing the keyspace_rf_change
and scheduling migrations.
We need to persist the ids of requests that are paused. A new column -
paused_rf_change_requests is added to system.topology table.
In this commit no data is kept in the new column.
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