Adding basic support for a user-interpretable resource label (#761)

* adding support for the user-interpretable label(UIR)

* more plumbing for num_uirs further upstream; set to infty when specified on cmd line

* pass default num_uirs for actors; update GlobalStateAPI

* support num_uirs in ray.init()

* local scheduler resource accounting: support num_uirs; prep for vectorized resource accounting

* global scheduler test updated

* Fix bug introduced by rebase.

* Rename UIR -> CustomResource and add test.

* Small changes and use constexpr instead of macros.

* Linting and some renaming.

* Reorder some code.

* Remove cpus_in_use and fix bug.

* Add another test and make a small change.

* Rephrase documentation about feature stability.

python/ray/utils.py

@@ -65,6 +65,7 @@ FunctionProperties = collections.namedtuple("FunctionProperties",
                                             ["num_return_vals",
                                              "num_cpus",
                                              "num_gpus",
+                                             "num_custom_resource",
                                              "max_calls"])
 """FunctionProperties: A named tuple storing remote functions information."""