[rllib] Cleanups: deep merge configs properly; enforce min iter time on APEX (#2500)

The dict merge prevents crashes when tune is trying to get resource requests for agents and you override a config subkey. The min iter time prevents iterations from getting too small, incurring high overhead. This is easy to run into on Ape-X since throughput can get very high.

python/ray/utils.py

@@ -267,13 +267,6 @@ def resources_from_resource_arguments(default_num_cpus, default_num_gpus,
     return resources
 
 
-def merge_dicts(d1, d2):
-    """Merge two dicts and return a new dict that's their union."""
-    d = d1.copy()
-    d.update(d2)
-    return d
-
-
 def check_oversized_pickle(pickled, name, obj_type, worker):
     """Send a warning message if the pickled object is too large.