storage: remove I/O operation in RowSetBuilder

[skyzh]

... and we need a new RowSetWriter to write what's inside builder to the disk.

[skyzh]

From my perspective, the RowsetBuilder should not contain any I/O related logic, and the logic like creating directory and flushing data should be moved to a separate module, maybe RowsetWriter? So that we won't need to create a directory even the rowset is still in memory.

Originally posted by @zzl200012 in #492 (comment)

From my perspective, the RowsetBuilder should not contain any I/O related logic, and the logic like creating directory and flushing data should be moved to a separate module, maybe RowsetWriter? So that we won't need to create a directory even the rowset is still in memory.

That's something I'd like to refactor :)

In fact, all I/O is done in flush_and_finish for RowSetBuilder. I plan to let the caller to decide whether to write these files or not, instead of doing all I/O inside the RowSetBuilder.

We already have a tracking issue for that #110

Originally posted by @skyzh in #492 (comment)

[zzl200012]

That's fine. I will take a look this weekend.

[skyzh]

Oops, it seems that #499 has already fixed this by introducing a RowSet writer. cc @zzl200012