Fix recovering desktop (FailoverManager)

Steps to Reproduce

1. Add in zk.xml

   <system-config>
   	<failover-manager-class>org.zkoss.zktest.test.DumbFailoverManager</failover-manager-class>
   </system-config>
   

2. Run zktest/test/failover.zul
3. Click the "kill desktop but recoverable" button.
4. Input "Tom" in the textbox.
5. Click the "Enter" button.

Current Result

Lots of exceptions. The desktop recovering was broken for a long time.

Expected Result

The message box with "Hi, Tom" should appear instead of a timeout warning.

Debug Information

The desktop ID is not started with "g" anymore.
After calling setId() in recovering, it would remove then add the desktop.
https://github.com/zkoss/zk/blob/v8.6.1/zk/src/org/zkoss/zk/ui/impl/DesktopImpl.java#L943,L950

When a desktop is destroying, _rque is null.
https://github.com/zkoss/zk/blob/v8.6.1/zk/src/org/zkoss/zk/ui/impl/DesktopImpl.java#L987

When destroying, the current execution will set as null even in recovering (which shouldn't in this case)
https://github.com/zkoss/zk/blob/v8.6.1/zk/src/org/zkoss/zk/ui/impl/SimpleDesktopCache.java#L169

Since this recovered desktop is not considered alive, there would be a lot of troubles.
https://github.com/zkoss/zk/blob/v8.6.1/zk/src/org/zkoss/zk/ui/impl/DesktopImpl.java#L980

Workaround