searchAndActivate: rework 'activity_count' parameter

- It's not deprecated anymore.
- When activities are grouped at CMFActivity level, it specifies a number of
  activities that are generated at a time instead of a number of activity
  groups.
- The special None value means no limit.

product/ERP5Catalog/CatalogTool.py

@@ -1349,26 +1349,33 @@ class CatalogTool (UniqueObject, ZCatalog, CMFCoreCatalogTool, ActiveObject):
       document returned by catalog) as first positional argument.
       Use 'packet_size' parameter to limit the size of each group (default: 30).
 
-      'activity_count' parameter is deprecated.
-      Its value should be hardcoded because CMFActivity can now handle many
-      activities efficiently and any tweak should benefit to everyone.
-      However, there are still rare cases where one want to limit the number
-      of processing nodes, to minimize latency of high-priority activities.
+      The maximum number of activities that are generated by this method
+      (before activating itself) can be tweaked with 'activity_count'.
+      Except as a way to limit the number of processing nodes, this should be
+      rarely used because CMFActivity can handle many activities efficiently
+      and we should rather have good default values.
+      The special None value means no limit, which can be useful when a catalog
+      search is so slow and doesn't return too many results.
+
+      'packet_size' is deprecated when used without 'select_method_id'.
       """
       catalog_kw = kw.copy()
       select_method_id = catalog_kw.pop('select_method_id', None)
+      limit = catalog_kw.pop('activity_count', 0)
       if select_method_id:
         packet_size = catalog_kw.pop('packet_size', 30)
-        limit = packet_size * catalog_kw.pop('activity_count', 100)
+        if limit is not None:
+          limit = packet_size * (limit or 100)
       elif 'packet_size' in catalog_kw: # BBB
         assert not group_kw, (kw, group_kw)
         packet_size = catalog_kw.pop('packet_size')
         group_method_cost = 1. / packet_size
-        limit = packet_size * catalog_kw.pop('activity_count', 100)
+        if limit == 0:
+          limit = 100 * packet_size
       else:
         group_method_cost = group_kw.get('group_method_cost', .034) # 30 objects
-        limit = catalog_kw.pop('activity_count', None) or \
-          100 * int(ceil(1 / group_method_cost))
+        if limit == 0:
+          limit = 100 * int(ceil(1 / group_method_cost))
       if min_uid:
         catalog_kw['min_uid'] = SimpleQuery(uid=min_uid,
                                             comparison_operator='>')