[dnsdist] pool selection without implicit accept

[Klaus Darilion]

Hello!

I have a ruleset with severals whitelist (AllowAction) and ratelimit
(MaxQPSRule+DropAction).

Now, independent of these rules I would like to use different backend
pools. But now I have a problem as PoolAction() immediately forwards the
request and my blacklist/whitelist rules are not handled anymore.

Moving the pool selection after the black/whitelist also does not work
as the whiteliste sends immediately to the default pool.

Hence, I suggest a PoolAction() without implicit AllowAction, for
example PoolActionSetOnly() or PoolActionContinue() to set the pool for
a request but continue in the rules processing.

regards
Klaus