net_transport.go
func (n *NetworkTransport) SetHeartbeatHandler(cb func(rpc RPC)) {
	n.heartbeatFnLock.Lock()
	defer n.heartbeatFnLock.Unlock()
	n.heartbeatFn = cb
}
func (n *NetworkTransport) SetHeartbeatHandler(cb func(rpc RPC)) {
	n.heartbeatFnLock.Lock()
	defer n.heartbeatFnLock.Unlock()
	n.heartbeatFn = cb
}
// SetHeartbeatHandler is used to setup a heartbeat handler
// as a fast-pass. This is to avoid head-of-line blocking from
// disk IO.