Gather four block/job status structures together (#3528)

This is part of work to make it easier to understand the four structures
and how they relate to each other.

This should not change any behaviour.

parsl/executors/status_handling.py

@@ -59,20 +59,28 @@ def __init__(self, *,
         else:
             self.block_error_handler = block_error_handler
 
-        # errors can happen during the submit call to the provider; this is used
-        # to keep track of such errors so that they can be handled in one place
-        # together with errors reported by status()
-        self._simulated_status: Dict[str, JobStatus] = {}
         self._executor_bad_state = threading.Event()
         self._executor_exception: Optional[Exception] = None
 
         self._block_id_counter = AtomicIDCounter()
 
         self._tasks = {}  # type: Dict[object, Future]
+
+        self._last_poll_time = 0.0
+
+        # these four structures track, in loosely coordinated fashion, the
+        # existence of blocks and jobs and how to map between their
+        # identifiers.
         self.blocks_to_job_id = {}  # type: Dict[str, str]
         self.job_ids_to_block = {}  # type: Dict[str, str]
 
-        self._last_poll_time = 0.0
+        # errors can happen during the submit call to the provider; this is used
+        # to keep track of such errors so that they can be handled in one place
+        # together with errors reported by status()
+        self._simulated_status: Dict[str, JobStatus] = {}
+
+        # this stores an approximation (sometimes delayed) of the latest status
+        # of pending, active and recently terminated blocks
         self._status = {}  # type: Dict[str, JobStatus]
 
     def _make_status_dict(self, block_ids: List[str], status_list: List[JobStatus]) -> Dict[str, JobStatus]: