to solve the issue #2246.
scheme is as following:
add a optional preferred_rowset_type in TabletMeta for V2 format rollup index tablet
add a boolean session variable use_v2_rollup, if set true, the query will v2 storage format rollup index to process the query.
test queries will be sent to online service to verify the correctness of segment-v2 by send the the same queries to fe with use_v2_rollup set or not to check whether the returned results are the same.
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