Add interface allowing partial updates to existing values

In WiredTiger there is currently no way to efficiently update a small portion of a large value, if not using WiredTiger schema layout. For example if a user is storing blobs of data that are 16k, and wants to update a single counter that forms part of the blob WiredTiger will rewrite the entire 16k value each time the value is updated.

It would be nice to be able to write only value deltas, and ideally to be able to efficiently retrieve such a delta directly from an application via the API.