Incremental encoding for GenericIndexed<String>

[leventov]

Delta encoding on the sorted sequence of strings could be applied: https://en.wikipedia.org/wiki/Incremental_encoding

It may significantly reduce size of high-cardinality string dimensions.

[leventov]

Based on https://arxiv.org/abs/1101.5506, decided to use Front Coding with the bucket size of 4.