API: add get estimated row size histogram to column family
API: add get estimated row size histogram to column family

This adds the implementation to in the API to the row size histogram.

It adds a map_cf method that perform a map operation over all column
family on the different shards.

Signed-off-by: Amnon Heiman <ed5fc74c4692051ad0b19749fd7cea6ec2d74b9e@cloudius-systems.com>

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