Rename on-connection-* into tunnel-* and simplify related code.

The names `on-connection-buffer` and `on-connection-process` are pretty confusing. They provide tunneling and are unlikely to be used for anything else, so let's name them as such. 

Some simplification/clarification of the code and documentation were made here and there and `nrepl-connection-endpoint` returns the process instead of the process' buffer-name. I think it's good to work with first principles whenever possible.

I have also increased the process-wait-for-output timout to .05. On some machines, keeping it very low makes Emacs pinging continuously up to the point that C-g  doesn't work anymore. This happens mostly on windows but people reported this on linux as well. It doesn't harm to have it higher in this particular  place. It's far from noticeable and the process returns faster on any input anyways. Better safe than sorry.