Filip Roséen authored Jul 17, 2018 and Romain Vimont committed Aug 20, 2018

As there are plans to make the src/misc/background_worker
multithreaded, the preparser must be changed in order to support
having several tasks running in parallel.

These changes accomplishes said goal by moving the relevant
task related data-members from struct input_preparser_t to a new one,
so that each task has its own state.