addressing genPOI RAM usage

I've had some trouble lately with running `overviewer.py --genpoi` resulting in an OOM-kill of our minecraft server. The problem seems to me the one described under #1090 1. Every Entity and TileEntity is read into memory and only later filtered. This should be changed and filtering should be done on the fly.

I have implemented a solution that works on my server. I haven't tested it anywhere else and I only generate signs, so manual POIs and Players are not tested, though that code was changed, too.

The first couple of commits are just some cleanups on my way to understanding, what the code does. Feel free to squash them, if you consider this code as good enough to be merged.