allow bridge mq cost msg

[hezhangjian]

motivation

allow bridgemq to cost the message

changes

add bool return value on Publish method to allow bridge mq to cost the message

type BridgeMQ interface {
	Publish(e *Elements) (bool, error)
}

If the bridgeMq returns true, we won't send msg to it's subscriber

[hezhangjian]

@chowyu08 PTAL, the conflict has been solved. thanks