Cherry-pick early-packet-in fix in POX debugger branch

[colin-scott]

The default STS config (fuzz_pox_fattree.py) is particularly good at triggering the "premature packet in" bug in POX, documented here: noxrepo/pox#108

It's a bit confusing to new users that they trigger a bug almost immediately in POX with the default config. The stacktrace is easily mistaken for a bug in STS, and users mistakingly believe they've screwed up STS installation as a result.

Murphy pushed a fix to this bug to dart. We should merge this into debugger branch.