Rename short request and response related variable names

[malud]

As we had received some feedback from users which have no related background to varnish, which inspired us to use bereq, beresp and req as names, we would like to refine this configuration part to be more expressive instead of the requirement looking up variable name specifications in the first place.

Following renames:
req -> request
ctx -> context
bereq -> removed
beresp -> removed
bereqs -> backend_requests
beresps -> backend_responses

bereq and beresp were shortcuts for beresps.default.var. There is no drop in replacement at the moment and we will discuss a possible default label shortcut later on. Accessing the default one: backend_responses.default.*.