Reuse temporary buffers when building an index

The temporary buffers when used for compression, and the uncompressed buffer could be reused between nodes.

temp should be easy. - construct with std::move and initialise if empty. swap at the end.
uncompressed is tricker because nodes are written to disk after the next leaf has been generated. Best would be to add a finialize() function to the node writer, and create the payload at that point.