* remove alpha_rowset_meta
* remove alpha rowset related codes in compaction
* remove alpha rowset related codes in RowsetMeta
* fix be ut because some ut use alpha rowsetmeta
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.
NOTE: This patch would modify all Backend's data.
And this will cause a very long time to restart be.
So if you want to interferer your product environment,
you should upgrade backend one by one.
1. Refactoring be is to clarify the structure the codes.
2. Use unique id to indicate a rowset.
Nameing rowset with tablet_id and version will lead to
many conflicts among compaction, clone, restore.
3. Extract an rowset interface to encapsulate rowsets
with different format.
