Compress blobs using lz4

A couple of complications with implementing, so not something to quickly slip into a release.

• Add a new createBlobNode function to the compressor.  (createNode has a different return type)
• Extend the compression interface to allow compress as much as you can into this space, and indicate how much was compressed.  (Useful elsewhere.)
• Up the version on the index.  Is this something just for use with inplace indexes or generally?