dnsdist: Speed up the processing of large ring buffers

[rgacogne]

Short description

• We kept track of the remote addresses for each StatNode entry, copying them around several times, while we never used them in dnsdist (dnsscope does)
• The exceed* functions (or more precisely the filterScore() one) sorted their return based on the score. We didn't used that (undocumented) property, and it led to unneeded copy of entire maps
• Use a reserve()'d unordered_map instead of a map for exceed* operations

Checklist

I have:

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