improve optimizer_use_condition_selectivity=5

• use it for more than LIKE
  • basically, it could be used for any condition that is too complex to be estimated without sampling
• use random dives, not table scan
• get rid of optimizer_selectivity_sampling_limit
  • either continue sampling until the result converges (using some hard-coded error marging and some safety check against sampling too much)
  • or, ideally, sample as long as needed:
    • sample a little bit, say, 10 rows. the selectivity will be within some range [sel _min; sel _max]
    • this corresponds to two execution plans, Plan _min with the cost Cost _min and a Plan _max with the cost Cost _max
    • we don't need more precise selectivity estimates as long as the best alternative plan is outside of [Cost _min; Cost _max]
    • otherwise we have to sample just enough to narrow down the range until it no longer includes the best alternative plan (or until the range converges and stops shrinking)