bugfix: the first UPDATE_MODEL task id should be selected carefully, so that all UPDATE_MODEL tasks could be taken out from workers' recved_msgs_
bugfix: the first UPDATE_MODEL task id should be selected carefully, so that all UPDATE_MODEL tasks could be taken out from workers' recved_msgs_

avantcredit/parameter_server,dmlc/parameter_server,hjk41/parameter_server,avantcredit/parameter_server,hjk41/parameter_server,austinbyers/parameter_server,ashhher3/parameter_server,xindaya/parameter_server,xindaya/parameter_server,dmlc/parameter_server,austinbyers/parameter_server,austinbyers/parameter_server,ashhher3/parameter_server,xindaya/parameter_server,xindaya/parameter_server,ashhher3/parameter_server,austinbyers/parameter_server,austinbyers/parameter_server,avantcredit/parameter_server,hjk41/parameter_server,hjk41/parameter_server,hjk41/parameter_server,ashhher3/parameter_server,dmlc/parameter_server,avantcredit/parameter_server,dmlc/parameter_server