ValueStore.Flush() now Puts all Chunks buffered in the ValueStore
layer into the underlying ChunkStore. The Chunks are not persistent
at this point, not until and unless the caller calls Commit() on
the ChunkStore.
This patch also removes ChunkStore.Flush(). The same effect can be
achieved by calling ChunkStore.Commit() with the current Root for both
last and current.
NB: newTestValueStore is now private to the types package.
The logic is that, now, outside the types package, callers
need to hold onto the underlying ChunkStore if they want to
persist Chunks.
Toward #3404
