debuginfod: PR30378: better compression for _files table

debuginfod: PR30378: better compression for _files table

Split the _files table into two links into a new _fileparts table,
interning the dirname and basename of each file name string.  This
reduces storage requirements for many thousands of almost-identical
long paths that are evident in large builds like kernels.

This is unfortunately a schema-breaking change, so requires reindexing
of the corpus.

While in the vicinity, the file scan work queue is changed from a
<set> to an <unordered_set>.  The intent is that files be scanned in a
more random sequence instead of sorted.  If they're sorted, then files
that contain errors will tend to be retried over and over again at the
next scan cycle, even at the expense of making progress on the other
files in the queue.

Signed-off-by: Frank Ch. Eigler <fche@redhat.com>