CMFActivity: do not refresh view too often when checking messages dependencies

The root call to getExecutableMessageList (i.e. the one from distribute)
is fast enough and won't hold old revisions of the database for too long.
It is also completely read-only so it won't lock anything.

This caused useless communication with the server.

product/CMFActivity/Activity/Queue.py

@@ -33,8 +33,6 @@ from zLOG import LOG, WARNING, ERROR
 from ZODB.POSException import ConflictError
 from cStringIO import StringIO
 
-import transaction
-
 # Time global parameters
 MAX_PROCESSING_TIME = 900 # in seconds
 VALIDATION_ERROR_DELAY = 15 # in seconds
@@ -114,7 +112,6 @@ class Queue(object):
     cached_result = validation_text_dict.get(message.order_validation_text)
     if cached_result is None:
       message_list = activity_tool.getDependentMessageList(message, self)
-      transaction.commit() # Release locks.
       if message_list:
         # The result is not empty, so this message is not executable.
         validation_text_dict[message.order_validation_text] = 0

product/CMFActivity/Activity/SQLBase.py

@@ -329,6 +329,7 @@ class SQLBase(Queue):
           message.order_validation_text = self.getOrderValidationText(message)
         self.getExecutableMessageList(activity_tool, message, message_dict,
                                       validation_text_dict, now_date=now_date)
+      transaction.commit()
       if message_dict:
         distributable_uid_set = set()
         serialization_tag_dict = {}