BUG/MEDIUM: server: remove immediately from GUID tree on delete server

BUG/MEDIUM: server: remove immediately from GUID tree on delete server

On "delete server", the target instance is removed from its various
attach points in the parent proxy and global collections. The server
will then be freed immediately or kept temporarily in the process until
its refcount is null.

In effect, this means that a server name can be reused immediately after
"delete server" success, even if the instance is not immediately freed.
This was not the case though with its GUID, as guid_remove() was
performed in srv_drop() on null refcount.

Changes this behavior so that guid_remove() is now called directly
during "delete server" handler. This is better as there is no difference
now for server names and GUIDs : both can be reused immediately after
the command completion.

This could be considered as a bug with a possible backport up to 3.0.
Currently it may be better to not pick it unless it becomes mandatory.