daemon: allow per-request variables in Lua

daemon: allow per-request variables in Lua

The handlers in Lua can now store per-request variables that are automatically
GC'd when the request is finished. This is useful for stateful modules,
such as DNS64 that uses internal option flags for state tracking.

The layers can now get a variable table like so:

```
local vars = kres.request_t(r):vars()
vars.hello = true
```

The variables are persisted between different layers for each request.