Improve ev_pickup events in tree mode on reroute

[thiell]

Lower priority but would be nice to fix ev_pickup events when a gateway is down and messages are rerouted in tree mode.

See test test_tree_run_abort_on_start in TreeWorkerTest.py where the check for ev_pickup is currently disabled.

The problem is in Worker/Tree.py:

    def _check_ini(self):
        self.logger.debug("TreeWorker: _check_ini (%d, %d)", self._start_count,
                          self._child_count)
        if self.eh is not None and self._start_count >= self._child_count:
            # this part is called once
            self.eh.ev_start(self)
            # Blindly generate pickup events: this could maybe be improved, for
            # example, generated only when commands are sent to the gateways
            # or for direct targets, using MetaWorkerEventHandler.
            for node in self.nodes:
                _eh_sigspec_invoke_compat(self.eh.ev_pickup, 2, self, node)

In addition the the problems highlighted by the inline comment, the call to ev_start() just before might have aborted the worker, and so ideally ev_pickup should not be generated in that case.