The Future class is now gone, and its functionality ("is the result
ready?", "give me the result value") is now folded into AsyncJob.  This
allows retrieving the result from multiple threads, with synchronization
handled in AsyncJob.  (This is more flexible than std::future allows, since
we don't need either move semantics or the ability to pass along exceptions
when the future is harvested.)

This fixes a number of race conditions that helgrind was flagging during
startup with complex scenes.  (Our prior use of std::future wasn't correct
in that we had unprotected access attempts from multiple threads.)