Optimized BugDir.uuids, caching on-disk bug uuids.

Optimized BugDir.uuids, caching on-disk bug uuids.

Output of
  python -m cProfile -o profile be list
  python -c "import pstats; p=pstats.Stats('profile'); p.sort_stats('cumulative').print_stats()"
on my slow netbook before optimization:
   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
    10290    0.417    0.000   10.832    0.001 libbe/bugdir.py:237(uuids)
after optimization:
      105    0.063    0.001    0.250    0.002 libbe/bugdir.py:237(uuids)
The old generator produced many more calls than the new
implementation, but the number of calls was not the source of the
slowdown (data not shown ;).