expired nodes: use better encoding #558
Description
The current encoding of expired nodes uses rlp for the path to the node.
This is a problem, because if a single leaf gets resurrected among many, the non-resurrected leaves' paths need to be updated, and that means the whole RLP has to be recomputed. This is quite inefficient. If there was only a byte prefix, the offset + size in the expired node record could simply be modified by incremeting the former and decrementing the latter.