Store less zip tree transition information

[faithokamoto]

Changelog Entry

To be copied to the draft changelog by merger:

• vg giraffe chaining mode (-b hifi, -b r10, -b chaining-sr) memory & speed optimizations; no effect on output

Description

This plucks out two optimizations I did in my MP chaining branch which can be easily inherited by the regular chaining.

• Instead of each transition storing graph distance and read distance, we store only indel distance.
• If a transition would be tossed out later because it fails the max_indel_bases threshold, then we don't save it in the first place.

Combined, these mean we store the absolute minimum amount of transition info necessary in order to run chaining (the memory part) and thus have to construct & sort less data (the speed part).