Preallocate Ray Workers (#62)

* Refactor to preallocate a pool of RayStage Actors

* corrected version of datafusion in requirements-in.txt

Author: robtandy

datafusion_ray/__init__.py

@@ -20,6 +20,6 @@
 except ImportError:
     import importlib_metadata
 
-from .core import RayContext, prettify, runtime_env
+from .core import RayContext, prettify, runtime_env, RayStagePool
 
 __version__ = importlib_metadata.version(__name__)