Fix high duration bucket starting time

Given 3600 bucket size, high duration bucket start times don't end with 0s where they should.

class Histogram {
  public:
    Histogram(std::chrono::seconds bucket_size);

  // iterator stuff, per bucket
};

// class?
struct Bucket {
  int count;
  std::chrono::seconds low, middle, high;
  operator int() const; // count
};

random_access_range and sized_range?

https://github.com/ericniebler/range-v3/blob/master/include/range/v3/range/concepts.hpp

We might need a transform iterator to wrap std::*maps iterator as Histogram.

Replace https://gitlab.com/signal9/angonoka/-/blob/development/src/predict.h#L26-31 with the new class.