Refactor parts of the periodic worker

Perform some adjustments to the periodic worker type to
make it easier to later change and make it better in the
future. These changes hopefully also reduce complexity
and increase understandability.

In part this deprecates providing a tombstone via
__init__() since that will restrict future enhancements we can
make (in retrospect we need more internal control over that
argument & type to build better workers).

Change-Id: I1965e157c303c2a45b9950e9f4a921c638f57fd1

taskflow/utils/misc.py

@@ -101,6 +101,12 @@ def countdown_iter(start_at, decr=1):
         start_at -= decr
 
 
+def reverse_enumerate(items):
+    """Like reversed(enumerate(items)) but with less copying/cloning..."""
+    for i in countdown_iter(len(items)):
+        yield i - 1, items[i - 1]
+
+
 def merge_uri(uri, conf):
     """Merges a parsed uri into the given configuration dictionary.