Reland "[lit] Synthesize artificial deadline"

We always want to use a deadline when calling `result.await`.  Let's
synthesize an artificial deadline (now plus one year) to simplify code
and do less busy waiting.

Thanks to Reid Kleckner for diagnosing that a deadline for of "positive
infinity" does not work with Python 3 anymore.  See commit:
4ff1e34b

I tested this patch with Python 2 and Python 3.

llvm-svn: 375165