R&D Scylla performance tunning

[tnozicka]

We need to do a research to figure out what part of the system we need to tune to get ScyllaDB a better performance. The output should be a design doc that lists which parameters need to be tuned for the node and for the container itself. In the next phase we'll decide on how to best adjust those - e.g. node-tuning-operator, using one of the upstream ones if it fits, adjusting container initialization depending on privileges, ...