[dnsdist] Keep Client IP across dnsdist and PDNSRecursor

[Daniel Stirnimann]

> The most clean example is when someone queries for a domain, dnsdist
> send the query to the recursor, the recursor gets a SERVFAIL and get
> back to the dnsdist something like.
> 
> Sending SERVFAIL to 127.0.0.1 during resolve of '58cl.com.' because:
> Too much time waiting for 58cl.com.|A, timeouts: 5, throttles: 0,
> queries: 7, 7898msec

dnsdist will always send queries to your pool servers using its own IP
address. So, you need to allow dnsdist to lookup domain names on your
server or else it gets a REFUSED answer or runs into timeout (in that
case the server is propably indicated as DOWN).

Daniel