Reduce `VirtualNodeCache` memory footprint

Memory overhead of `VirtualNodeCache` comes from the following sources:

- `ConcurrentHashMap$Node`s of three maps
- `Mutation` fields (`key`, `value`, `deleted`, `version`)
- three `ConcurrentArray`s keeping extra references to `Mutation`s and forcing them to maintain a separate `key`

An ad hoc implementation of a concurrent map may significantly reduce data redundancy without sacrificing performance.
Although the current design assumes only one writer with multiple concurrent readers, the implementation offloads concurrency control to `ConcurrentHashMap` and is robust against concurrent updates (`VirtualRootNode` employing `VirtualNodeCache` is not). It would be good to preserve that.

Another requirement is implementation of parallel map entry processing without incurring more memory overhead as is the case with `ConcurrentHashMap`.