As discussed offline, the new node-change mode could be used for
evacuation, but it's not directly useful as it returns a list of
opcodes; therefore, we need to partially revert commits fbe5fcf6 and
5b53ca79

 that removed it (and multi-evacuate, which remains removed).

The new version of relocate is actually just a wrapper over the
tryNodeEvac (which does the node evacuate); we run that and then we do
some extra checks that the nodes we got from that function are
consistent with the instance's new state.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>