Cap time until full at 1 billion seconds

JSON can't represent infinity (at least not portably) so we need to use
a finite value. I certainly won't be worried if a disk will fill up in
32 years.

process_queue

@@ -35,7 +35,7 @@ class DiskFullPredictor:
             return
         current_used_bytes = lts.data[-1][1]
         current_usable_bytes = usable_lts.data[-1][1]
-        tuf = float('inf')
+        tuf = 1E9
         for d in reversed(lts.data):
             if d[1] < current_usable_bytes * 0.1:
                 continue # for sanity