Bug: client hangs in federation mode when connection between nodes broken.

[DrmagicE]

In federation mode, when the TCP connection between nodes gets broken, (e.g. pull the cable out), all clients will hang until the client-side close the connection.
Here is the culprit:

gmqtt/plugin/federation/peer.go

Lines 173 to 186 in 0a2b151

	func (p *peer) stop() {
	select {
	case <-p.exit:
	default:
	close(p.exit)
	}
	p.stateMu.Lock()
	if p.state == peerStateStreaming {
	_ = p.stream.client.CloseSend()
	}
	p.state = peerStateStopped
	p.stateMu.Unlock()
	p.stream.wg.Wait()
	}

CloseSend() will not close the underlying TCP connection of the gRPC streaming, it will wait for the response from the server and then close the connection.

if p.state == peerStateStreaming { 
 		_ = p.stream.client.CloseSend() 
} 

This will cause p.stream.wg.Wait() to hang until the connection of the gRPC streaming timeout.

Add a cancelable context.Context should fix the problem.

related to #121
reference:
grpc/grpc-go#2015 (comment)