There were several problems in the previous implementation. Now:
 - We handle separetly numbers of nucleotides in seq_{left,right} and in ref_{left,right};
 - There is no need to handle equality cases (if is the responsability of the segment_cost);
 - There is no need anymore to call dp.backtrack() (we directly use dp.best_score_on_i).
 - We also properly update del_D_left and del_D_right for VDJ recombinations

The tests added in 3c138657 are now passing.