[xray] raylet scheduling mechanism with a simple spillback policy (#2749)

## What do these changes do?
* distribute load and resource information on a heartbeat
* for each raylet, maintain total and available resource capacity as well as measure of current load
* this PR introduces a new notion of load, defined as a sum of all resource demand induced by queued ready tasks on the local raylet. This provides a heterogeneity-aware measure of load that supersedes legacy Ray's task count as a proxy for load.
* modify the scheduling policy to perform *capacity-based*, *load-aware*, *optimistically concurrent* resource allocation
* perform task spillover to the heartbeating node in response to a heartbeat, implementing  heterogeneity-aware late-binding/work-stealing.

test/actor_test.py

@@ -766,9 +766,6 @@ class ActorsWithGPUs(unittest.TestCase):
 
     @unittest.skipIf(
         os.environ.get('RAY_USE_NEW_GCS', False), "Crashing with new GCS API.")
-    @unittest.skipIf(
-        os.environ.get("RAY_USE_XRAY") == "1",
-        "This test does not work with xray yet.")
     def testActorGPUs(self):
         num_local_schedulers = 3
         num_gpus_per_scheduler = 4
@@ -812,9 +809,6 @@ class ActorsWithGPUs(unittest.TestCase):
         ready_ids, _ = ray.wait([a.get_location_and_ids.remote()], timeout=10)
         assert ready_ids == []
 
-    @unittest.skipIf(
-        os.environ.get("RAY_USE_XRAY") == "1",
-        "This test does not work with xray yet.")
     def testActorMultipleGPUs(self):
         num_local_schedulers = 3
         num_gpus_per_scheduler = 5
@@ -887,9 +881,6 @@ class ActorsWithGPUs(unittest.TestCase):
         ready_ids, _ = ray.wait([a.get_location_and_ids.remote()], timeout=10)
         assert ready_ids == []
 
-    @unittest.skipIf(
-        os.environ.get("RAY_USE_XRAY") == "1",
-        "This test does not work with xray yet.")
     def testActorDifferentNumbersOfGPUs(self):
         # Test that we can create actors on two nodes that have different
         # numbers of GPUs.
@@ -982,9 +973,6 @@ class ActorsWithGPUs(unittest.TestCase):
         assert ready_ids == []
 
     @unittest.skipIf(sys.version_info < (3, 0), "This test requires Python 3.")
-    @unittest.skipIf(
-        os.environ.get("RAY_USE_XRAY") == "1",
-        "This test does not work with xray yet.")
     def testActorsAndTasksWithGPUs(self):
         num_local_schedulers = 3
         num_gpus_per_scheduler = 6