Revert "Products.CMFActivity.Activity.SQLBase: Drop "FOR UPDATE"."

This reverts commit 89aa2a6b.
The assumption that FOR UPDATE was unnecessary is wrong: when SQLDict
reserves similar activities, it will race against these primo-reservations.
So this change actually opens the possibility for duplicate activity
reservation.
Revert this change until (hopefully) a better fix is implemented.

product/CMFActivity/Activity/SQLBase.py

@@ -774,7 +774,8 @@ CREATE TABLE %s (
           b"  processing_node=0 AND"
           b"  %s%s"
           b" ORDER BY priority, date"
-          b" LIMIT %i" % args,
+          b" LIMIT %i"
+          b" FOR UPDATE" % args,
           0,
         ))
       else:
@@ -787,6 +788,7 @@ CREATE TABLE %s (
           b"  %s%s"
           b" ORDER BY priority, date"
           b" LIMIT %i"
+          b" FOR UPDATE"
         b")" % args).format
         result = Results(query(
           b"SELECT *"