Introduce the abstraction of RunHandler which each DirectSession can use for

the duration of a single RunInternal() call from RunHandlerPool. It is used for
running inter-op closures with a global scheduler (which in the future) to
improve both median and tail latency (for use-cases like CPU inference).
In the case that global pools aren't used, this change should be a no-op.

PiperOrigin-RevId: 214992852