artmap

@petermattis @petermattis we took a lot of inspiration from Pebble, and even borrowed surrealdb/arenaskiplist to improve performance beyond what we were able to get initially (attribution to Andy Kimball is at the bottom of the README). We ended up settling on a surrealdb/artmap for that data-structure instead, because we were able to get better performance in Rust for the concurrency. One thing - this is a relatively small benchmark of course, because when comparing LMDB/libmdbx and the other rust based LSM trees it took a very long time to run the benchmarks, but we will run a much larger benchmark comparing just RocksDB and SurrealKV.