the num segments should be read from rowset meta pb.
But the previous code error caused this value not to be set in some cases.
So when init the rowset meta and find that the num_segments is 0(not set),
we will try to calculate the num segments from AlphaRowsetExtraMetaPB,
and then set the num_segments field.
This should only happen in some rowsets converted from old version.
and for all newly created rowsets, the num_segments field must be set.
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