Sequence chunker should avoid eagerly persisting root chunk

[ghost]

Now that prolly trees are eagerly written on creation, the sequence chunker chunker should avoid persisting the root chunk (which is returned). This is important, because the root chunk may not be directly addressable in the resulting value (i.e. if it is embedded into another value).

[kalman]

Started looking at this. It isn't trivial when you consider all of the special cases listed at https://github.com/attic-labs/noms/blob/759cf4fe589dd8a0af4e08983637c8b10cff29cc/go/types/sequence_chunker.go#L204 namely that we don't really know what the root chunk is until Done is called.

Oh well, here goes...