Reference : Make m_plugEdits key more unique

On Windows in particular, and it's conceivable that it could happen on
Linux, `plugEdit()` would occasionally find an edit for a plug where
none should exist. This was due the memory address of the plug, used as
a key for `m_plugEdits`, for a newly created plug was the same as a
recently deleted plug.

This was causing a test failure in
`GafferTest.ReferenceTest.testAddingChildPlugs` on Windows with
something like 1 : 1000 probability.

src/Gaffer/Reference.cpp

@@ -312,15 +312,15 @@ class Reference::PlugEdits : public Signals::Trackable
 			int64_t sizeAfterLoad = -1; // Default value means size not tracked
 		};
 
-		std::unordered_map<const Plug*, PlugEdit> m_plugEdits;
+		std::unordered_map<ConstPlugPtr, PlugEdit> m_plugEdits;
 
 		const PlugEdit *plugEdit( const Plug *plug ) const
 		{
 			// Cheeky cast better than maintaining two near-identical functions.
 			return const_cast<PlugEdits *>( this )->plugEdit( plug, /* createIfMissing = */ false );
 		}
 
-		PlugEdit *plugEdit( const Plug *plug, bool createIfMissing )
+		PlugEdit *plugEdit( ConstPlugPtr plug, bool createIfMissing )
 		{
 			if( plug->node() != m_reference )
 			{
@@ -333,7 +333,7 @@ class Reference::PlugEdits : public Signals::Trackable
 				return &(it->second);
 			}
 
-			if( !m_reference->isReferencePlug( plug ) )
+			if( !m_reference->isReferencePlug( plug.get() ) )
 			{
 				// We'll allow retrieval of existing edits on this plug, but we
 				// won't create new ones.