Fixed marker cache to handle asynchronous arrivals to disposed maps.
Because icon generation is now asynchronous, it's possible for an icon
to arrive after the map that wants it, and its cache, has been
disposed.  Which caused a write into a nonexistent cache.  Instead, we
now ignore the request.

Presidents exhibit was broken.  The auto-position option wasn't
working.  Investigation suggests it hasn't been working for a very
long time---there's a 2 year old bug in the code.  For now, I've
removed use of that option.