grid: Simpler reconnect logic (#18889)

Do not rely on `connChange` to do reconnects.

Instead, you can block while the connection is running and reconnect 
when handleMessages returns.

Add fully async monitoring instead of monitoring on the main goroutine 
and keep this to avoid full network lockup.

internal/grid/muxclient.go

@@ -533,7 +533,9 @@ func (m *muxClient) closeLocked() {
 	if m.closed {
 		return
 	}
-	close(m.respWait)
-	m.respWait = nil
+	if m.respWait != nil {
+		close(m.respWait)
+		m.respWait = nil
+	}
 	m.closed = true
 }