dnsdist: Enforce concurrent streams count for pending queries

dnsdist: Enforce concurrent streams count for pending queries

The gist of the `MadeYouRest` attack is that streams can be reset
by the client, and thus no longer count towards the maximum number
of a streams as far as the library is concerned, while the server
is still processing the query and doing actual work.
This pull request introduces a counter for "killed but still being
processed streams" to prevent it.

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