Use rolling hash splitter (data sensitive, e.g. Buzhash or Rabin) by default instead of SizeSplitter (data insensitive)

[sideeffffect]

Use the Buzhash or Rabin splitter in IPFS instead of the SizeSplitter by default.
A rolling hash based data sensitive splitter has a huge obvious advantage: it creates shift-resistant chunks and thus improves data deduplication and sharing.

For example casync is doing it too:

• http://0pointer.net/blog/casync-a-tool-for-distributing-file-system-images.html
• https://github.com/systemd/casync
• https://github.com/folbricht/desync (alternative Go implementation)

Original ticket: ipfs/go-ipfs-chunker#13