Strip out mmap tooling and simplify polling function

I was reading through the tests and realised that this mmap stuff was making things awkward, it actually serves no purpose in the hevclient. It's a legacy workaround for the webui developers when using flask in developer mode, but really that doesn't belong in the hevclient base class.

I've stripped out the mmap stuff and gone back to the base hevclient. I have also made some readability changes in the hevclient polling function while keeping the functionality unchanged.