Add HTTP headers to an optional kr_request.qsource.headers structure.
Headers are stored as name, value string pairs.

The following snippet can be used to access the headers in lua modules:

```
if (req.qsource.headers ~= nil) then
	for i = 1, tonumber(req.qsource.headers.len) do
		local name = ffi.string(req.qsource.headers.at[i - 1].name)
		local value = ffi.string(req.qsource.headers.at[i - 1].value)
		print(name, value)
	end
end
```

Fixes #616