streamthinca: improve lower_bound_in_seglist()

- a thinko was causing it to include one too many segments below the bound
  than required to satisfy the criterion.  it wouldn't lead to incorrect
  results, just a small slow-down.  ligo.segments now provides an
  implementation of this function, but that version of ligo.segments is not
  yet in production so we need to retain this implementation here for the
  time being.