Implemented as a XFR handler task, in a same way as NOTIFY queries
for example. It also allows to change TSIG keys between hops,
or no keys at all - this clear AD flag.
Like: client <--key:A--> f1 <--key:B--> f2 <--NOKEY--> MASTER
Forwarded queries also keep socket mode.

To do:
According to RFC2845, it should also forward queries with no
known key intact to the next forwarded, this is not done yet.

refs #1130