dnsdist: Make inserting to the in-memory rings a bit faster

dnsdist: Make inserting to the in-memory rings a bit faster

This commit moves the allocation and copy of the DNS name before
taking the lock, reducing contention. In completely unrealistic
benchmarks this makes the insertion ~10% faster.
Ideally I would rather move the existing `DNSName` instead of allocating
a new one, as we are usually done with it by the point we insert
into the rings, but this involves a lot of changes so let's start
with this.

Signed-off-by: Remi Gacogne <remi.gacogne@powerdns.com>