Some more locking related changes

Hi!

I just worked a bit on the CalendarTool to remove unnecessary locks in
_findCache (and removed some more even more unnecessary locks - only write
access and internal structure changes should be blocked with locks), so now
the CalendarTool should produce much less lockings and so shouldn't be too
much of a problem. Before this change the CalendarTool was the main source
for active locks in PyDS.

bye, Georg