A way to speed up the preparsing consists in preparsing several inputs
in parallel.

For this purpose, make the background worker (used by the preparser and
fetcher) execute tasks from (possibly) several threads.

Apart from adding a new field "max_threads" in the
background_worker_config structure, the background worker API is kept
unchanged.

Two new options are added to configure the maximum number of threads
used for preparsing and fetching:
 - preparse-threads
 - fetch-art-threads