BetaRowsetWriter is used to write rowset in V2 segment format.
This PR contains several interface changes
1. Rowset.make_snapshot() is renamed to `link_files_to` because hard links are also useful in copy task, linked schema change, etc
2. Rowset.copy_files_to_path() is renamed to `copy_files_to` to be consistent with other names
3. RowsetWriter.mem_pool() is removed because not all rowset writers use MemPool
4. RowsetWriter.garbage_collection() is removed because it's not used by clients
5. SegmentGroup's make_snapshot() is removed because link_segments_to_path() provides similar functionality
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.
