Mock histogram on router

This is the first step to implement the cost-based JOIN optimization for the distributed SQL queries. At the very beginning we need a playground to verify our theory, that PostgreSQL-like equi-height histograms with MCV are be good enough to help us for all cases. As a bonus this issue would help us to implement a test framework for the cost-based optimizations in SBROAD before we'll start using it for the real queries.

The histogram space should be mocked in the Rust memory of the sbroad's test runtime. Also it would be nice if its format would be compatible with PostgreSQL pg_stats view format.