Mux: Send async mux ack and fix stream error responses (#19149)

Streams can return errors if the cancelation is picked up before the response 
stream close is picked up. Under extreme load, this could lead to missing 
responses.

Send server mux ack async so a blocked send cannot block newMuxStream 
call. Stream will not progress until mux has been acked.

internal/grid/connection.go

@@ -1504,6 +1504,9 @@ func (c *Connection) handleMuxServerMsg(ctx context.Context, m message) {
 		})
 	}
 	if m.Flags&FlagEOF != 0 {
+		if v.cancelFn != nil && m.Flags&FlagPayloadIsErr == 0 {
+			v.cancelFn(errStreamEOF)
+		}
 		v.close()
 		if debugReqs {
 			fmt.Println(m.MuxID, c.String(), "handleMuxServerMsg: DELETING MUX")