dnsdist: Implement ContinueAction()

[rgacogne]

Short description

As discussed on the mailing-list 1, this new action makes it possible to use an otherwise terminal action without stopping the rule processing. For example:

addAction(AllRule(), PoolAction("mypool"))
addAction(AllRule(), DisableValidationAction())

would route all incoming queries to the mypool pool, and since the PoolAction() terminates the processing the DisableValidationAction() one would never be applied.
With the new action, it is possible to do:

addAction(AllRule(), ContinueAction(PoolAction("mypool")))
addAction(AllRule(), DisableValidationAction())

This would result in all queries being routed to the mypool pool with the CD flag being set.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[zeha]

will this cause incorrect stats?

[rgacogne]

If you do ContinueAction(DropAction()) then yes, but IMHO you then deserve it :)