dnsdist: Replace the Lua params with a DNSQuestion `dq` object

dnsdist: Replace the Lua params with a DNSQuestion `dq` object

In order to:
1. Be able to add functions/member without breaking the API
2. Being as compatible as possible with the PowerDNS Lua API

To limit the parsing/copy to a minimum, this DNSQuestion differs
from the PowerDNS one. Most Lua members are properly wrapped,
but it currently lacks some advanced functions like `getRecords()`
or `setRecords()`, that we might add later.
In addition to the existing `tostring()`, this commit adds
`toString()` ones to match the PowerDNS syntax.

LuaWrapper is supposed to support read-only members, where you
only define the getter and no setter, but I can't find the right
syntax for that to work, so for now the setter are present for
read-only members, and just do nothing.