dnsdist: Remove 'expired' states from MaxQPSIPRule

[rgacogne]

Short description

The MaxQPSIPRule rule needs to keep a state for seen source addresses. While this state is small we don't want to keep it for too long so this PR adds a cleaning mechanism that will remove entries that haven't been seen for a while. It also reduces the size of the state struct.

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)