Moved the code that checks for finished file-lists, for finished "redo"

Moved the code that checks for finished file-lists, for finished "redo"
sends, and finished hard-links into check_for_finished_files(), a new
function that takes the place of check_for_finished_hlinks().  This
lets us signal the sender more rapidly when a file-list is done (which
ensures that it sends us more work to do in a timely manner), and also
avoids two potential deadlocks in the old code.