major change (details below): reload is now careful of running provers and focused node

The decision on clearing the whole tree and sending again all nodes
is now completely handled on the itp server side.

- discarded the request Get_Session_Tree_req
- added the notification Reset_whole_tree

Bad side effect: moving to next unproven goal seems working even worse than before

missing feature: reloading when a node is focused should keep the focus
if possible, for now it just unfocus