fixing a (likely) race condition when simultanous pickling/unpickling can leave

inconsistent memo states.  Rather than adding locks, the fix actually simplifies the code by
keeping the pickling/unpickling memo's in sync directly and more efficiently, also removing
a long outstanding XXX.

testing/test_mypickle.py

@@ -252,10 +252,3 @@ class TestPickleChannelFunctional:
         channel.waitclose(timeout=2)
 
 
-
-def test_explode():
-    from xdist.mypickle import explode
-    assert isinstance(explode({}.values()), list)
-    l = []
-    assert isinstance(explode(l), list)
-    assert explode(l) is l