Make BugDir._uuids_cache a set.

Make BugDir._uuids_cache a set.

For `be list` on a bugdir with 4096 open bugs, this reduced the
cumulative time spend in 8194 calls to BugDir.uuids() from 41 seconds
to 33 seconds.

Of the 33 cumulative seconds, 24 were spend in uuids() itself (and not
in child functions), which is probably from the list comprehension
extracting in-memory Bug uuids.  With fancier accounting, you could
probably trust _uuids_cache to already contain all the in-memory
uuids and dispense with the union altogether.