preparser and fetcher: decrease executor(=thread) count

On top on !6142 (merged), !6149 (merged), and !6150 (merged)

By default, a single preparser is creating 4 threads: one for parsing, 3 for fetching (local, network, download).

vlc_preparser_New now ask for the type of parsing/fetching to set up the executors accordingly and avoid creating threads that won't be used.

It is also now possible to use the preparser to only fetch metadata, without going through the parser executor.