LinkRefPair GUID could change due to garbage collection

The GUID stored on the LinkRefPairObjectFactory class could change if this class gets garbage collected.

This could be case when CallByValue=true on the naming service since the LinkRefPairs are serialized
with any hard reference to the classes lost.

Even if this is not the cause of the problem, it is still a good idea to make sure this class
doesn't get garbage collected while there are active LinkRefPairs otherwise the GUID will change
and the local/remote mechanism won't work properly.

To fix this, the LinkRefPairService should hold a static reference to the ObjectFactory class.