cxxll::parallel: New class
symboldb_download_repo: Refactor, introduce downloader::download_main_task
symboldb_download_repo: Use cxxll::parallel
cxxll::dwarf::cu_range: Add support for linking alternate debuginfo

This adds members debugaltlink_buildid(), set_alt().
rpm_load: Delay ELF loading until all ELF files have been seen
rpm_load: Locate and merge debugaltlink DWARF data
rpm_load: Include all ELF files in the cache, not the ones just loaded
database::add_dwarf(): Filter more compilation units without useful data
fixup cu_range