greenio: When we mark a socket as closed we still need to be able to close it

https://github.com/eventlet/eventlet/issues/122

After a long running job that highlighted a previous known issue with
Second Simultaneous Read issue I used http://mg.pov.lt/objgraph/ to print
out all the types and counts still referenced by the GC.

Before this patch I was seeing GreenSocket and _socketobject types been
referenced. Depending on how long I ran the test job for I saw more references
to these objects. After this patch I see no reference no matter how long I
run my test for.