Currently, we have Field and ColumnSchema to access column data in a
row. These two classes are mostly the same. So we should unify these to
one class. Now, Field has offset information, which is an row attribute,
so we remove offset in Field.
RowCursor now has some logic which belong to Schema, so in this patch I
add Schema attribute to RowCursor to make RowCursor simple. After this
change, only Schema will handle Field/ColumnSchema.
I extract some logic from RowCursor to be/src/olap/row.h, then we can
use same logic to handle different types of row. Each type of row has
same function that to get Cell of this row. A cell represent a column
content with a null indicator.
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.
