Fix application:env and stoping/starting related issues

This seems to work nicely.

However, I am a bit unhappy about the fact that there's a cyclic dependency between `piqi_rpc` and `piqi_rpc_http` modules. The fact that piqi-rpc listeners live in a different supervision tree opens up some more possibilities for race conditions, when adding/removing services and/or crashing, so it would be good to rethink this from scratch, but I'm too braindead for the moment to do it, so just pushing this out with a hope that someone else will spot any obvious issues if they're out there.