Unified parallel GBWT construction

[jltsiren]

Changelog Entry

To be copied to the draft changelog by merger:

• Multithreaded path cover / local haplotypes GBWT construction.

Description

This PR updates GBWTGraph to include unified support for multithreaded GBWT construction (see jltsiren/gbwtgraph#36). The main idea is to partition the graph into GBWT construction jobs using gbwt_construction_jobs(), create the final metadata with MetadataBuilder, pass reference paths with assign_paths() and insert_paths(), build partial GBWTs for the jobs in parallel, merge the GBWTs, and add metadata from MetadataBuilder.

• Path cover / local haplotypes GBWT construction uses this now, both in vg gbwt and vg autoindex. As a byproduct, we get parallelization for them.
• GBWT/GBZ construction from GFA already used this.
• Haplotype sampling in vg haplotypes uses this to the extent possible.
• rebuild_gbwt() still uses the old parallelization scheme, as it's built into the interface.