dev-460: disallow a cigar 20M2D2I30M in extension

Global alignment does not allow contiguous insertions and deletions, but local
alignment and extension allow such CIGARs. The optimal global alignment may
have a lower score than extension, which actually happens often for PacBio
data. This commit disallows a CIGAR like 20M2D2I30M to fix this inconsistency.
Local alignment has not been changed.
dev-461: added a heuristic for PacBio data

See the comment above mem_test_chain_sw() for details.