rfc27: add allocated key to hello response

Problem: when resources are partially allocated across a scheduler
reload, the HELLO protocol has no way to inform the scheduler
which resources of the job are allocated or freed.

Add an optional "allocated" idset key to the response.  If the key
is present, only the ranks in the idset should be considered allocated.

See also: flux-framework/flux-core#6089

spec_27.rst

@@ -153,6 +153,13 @@ userid
 t_submit
   (double) job submission time
 
+and the following OPTIONAL key:
+
+allocated
+  (string) An RFC 22 idset representing the ranks (execution targets)
+  of this job's resource set that are currently allocated.  If this key
+  is omitted, the scheduler SHALL assume that all of R is currently allocated.
+
 Example:
 
 .. code:: json
@@ -162,6 +169,7 @@ Example:
      "priority": 43444,
      "userid": 5588,
      "t_submit": 1552593348.073045,
+     "allocated":"1-16,18",
    }
 
 For each job response, the scheduler SHALL mark its assigned resources