Better debug and logging for data-plane-gateway

[psFried]

data-plane-gateway is still limping along with PrintF logging :crying_cat_face: and there's a few other pieces we should address:

• add a real logging package, so that we can enable more verbose logging to help debug issues
• Add debug log statements where in the obvious places where they'd be useful (e.g. dispatching grpc requests)
• Wire up the /debug/pprof endpoint on the plain HTTP listener so we can get goroutine traces, etc.

[psFried]

This was completed in #21