debuginfod: PR25978 - Created the prefetch fdcache

debuginfod: PR25978 - Created the prefetch fdcache

The debuginfod fdcache-prefetch logic has been observed to show some
degeneracies in operation.  Since fdcache evictions are done
frequently, and freshly prefetched archive elements are put at the
back of lru[], each eviction round can summarily nuke things that
were just prefetched .... and are just going to be prefetched again.
It would be better to have two lru lists, or being able to insert
newly prefetched entries somewhere in the middle of the list rather
than at the very very end.

https://sourceware.org/bugzilla/show_bug.cgi?id=25978

Signed-off-by: Noah Sanci <nsanci@redhat.com>