Add some of the ideas from the START paper: multilevel nodes etc.

[rdaum]

The Self-Tuning Adaptive Radix Tree paper (https://db.in.tum.de/~leis/papers/artsync.pdf) adds some new concepts, which I might attempt to add to the existing implementation.

• A "tuning" pass
• A version of Node4 which is "multilevel"
• "Rewired" nodes via kernel page remapping

I want to look into see how much of this can be pulled into this existing implementation.

C++ implementation here for reference: https://github.com/jungmair/START