Raw import pdata replacement structure.

Credit to Vincent Pelletier.
Initialize video bt5
Initialize log bt5
Plug the video to the DMS
Use the BigFile document to allow uploading big data.
Rename file.
Drop debugging prints.
Drop offset-related assertions.

LOBTree raises when a key is out of range.
Drop unused/redundant locals.
Delay StringIO creation until actually needed.
Drop unused import.
Use a pseudo-private property for the chunk tree.
Rework BTreeData class.

Add inline test cases.
Fix several bugs brought up by test cases.
Alter PersistentString instances when they already exist rather than
creating new ones.
Fix len() calls on chunks.

Add a test case for one.
Prevent using negative offsets.
Expose a storage-efficient way to access a large area.
Limit the amount of memory used when generating padding over sparse areas.
Support streaming big file in DMS.
Directly display the video in the page
Allow to upload in chunk.

Data can only be appended.