[bloodstone] bring back deltas for random write friendliness

[spacejam]

for each node, bring back the stable sled bw-tree-esque deltas for improving random write performance. use a bounded fst-map for each node that points to heap locations for each value. Have keys in the fst map be implicitly prefixed by the low key of the node to further reduce memory usage. Basically turns it into a per-node index+log that buffers writes into the bulkier storage node.