Now, an agent connect statement can come before application attach-agent
which complies with NS. In addition, the order of
"$ns attach-agent <node> <agent>" and "$ns connect <agent1> <agent2>"
can by anything now. Reversing the order in NS causes problems for this though.
However, in emulab's case, we don't need that order. Most of these checks
are now moved to updatedb reducing some clutter in traffic.tcl

One thing that NS supports and it would be difficult for us is the order
of application attach-agent and "$ns at" statements. In NS, its ok to
flip the order. In emulab, its still not. Thats coz "$ns at" statements
need to know what vnode is the event going to. Of course, this can be
fixed by making "$ns at" storing the eventstring as is and only parsing
during the updatedb/$ns run phase. Seems like its not worth changing now.