Will be used by tablet-based replication strategies, for which
effective replication map is different per table.
Also, this patch adapts existing users of effective replication map to
use the per-table effective replication map.
For simplicity, every table has an effective replication map, even if
the erm is per keyspace. This way the client code can be uniform and
doesn't have to check whether replication strategy is per table.
Not all users of per-keyspace get_effective_replication_map() are
adapted yet to work per-table. Those algorithms will throw an
exception when invoked on a keyspace which uses per-table replication
strategy.
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