[Improvement] Better parallel management

Parallelization is implemented using pthreads, and each thread is affected to a contiguous partition of the file. This is prone to unbalanced workload if the file is ordered by some property.

We would like to distribute the work more evenly, for example by using a job queue. OpenMP is probably a good way to do that quite simply, but would need a rewrite of a big part of the code.