Refactor SerializationFormat::EndWrite and ChunkedIO::Chunk mem mgmt.

SerializationFormat::EndWrite now transfers ownership of the buffer
to the caller instead of doing a memcpy.

ChunkedIO::Chunk is no longer a POD type, hopefully the ctor/dtor
make it easier to manage its associated memory.  It also now
tracks how to deallocate its buffer (i.e. delete vs. free).