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.

src/common/format/common.fbs

@@ -9,9 +9,11 @@ enum ResourceIndex:int {
   CPU = 0,
   // A graphics processing unit.
   GPU = 1,
+  // A user-defined custom resource.
+  CustomResource = 2,
   // A dummy entry to make ResourceIndex_MAX equal to the length of
   // a resource vector.
-  DUMMY = 2
+  DUMMY = 3
 }
 
 table Arg {