Make get_thread virtual in Node class

[med-ayssar]

The get_thread function in the Node class is not virtual, which makes the implementation of the function in the ProxyNode class useless. Mainly, the get_thread function in the ProxyNode class will never be called, as we always have a pointer of Node type pointing to the proxy instance.

My suggested change is to make the function virtual and remove the implementation from the ProxyNode.

[jougs]

As discussed offline, this changes are problematic in that they

1. add a vtable indirection for all calls to get_thread(), which might lead to a performance regression
2. shadow the fact that get_thread() must not be called on a proxynode as it cannot return a sensible value for remote nodes represented by it.

I will thus close it.