bugfix: caused by the latest change
r323: added Z-dropoff, a variant of blast's X-drop
r324: a little code cleanup

The changes after r317 aim to improve the performance and accuracy for very
long query alignment. The short-read alignment should not be affected. The
changes include:

1) Z-dropoff. This is a variant of blast's X-dropoff. I orginally thought this
   heuristic only improves speed, but now I realize it also reduces poor
   alignment with long good flanking alignments. The difference from blast's
   X-dropoff is that Z-dropoff allows big gaps, but X-dropoff does not.

2) Band width doubling. When band width is too small, we will get a poor
   alignment in the middle. Sometimes such alignments cannot be fully excluded
   with Z-dropoff. Band width doubling is an alternative heuristic. It is based
   on the observation that the existing of close-to-boundary high score
   possibly implies inadequate band width. When we see such a signal, we double
   the band width.