debuginfod: rework threading model for file/archive scanning

debuginfod: rework threading model for file/archive scanning

We switch from a thread per supplied PATH, with a semaphore based
concurrency control, to a fixed number of worker threads collecting
the result of a plain directory traversal being put into a work queue.
This allows maximal continuous concurrency, even if the PATH
directories are dramatically differently sized.  There is no more need
to use concurrency-motivated subdirectory wildcards for PATH entries:
just a single top level directory will work fast.  doc & tests incl.