dnsdist: Only allocate the health-check mplexer when needed

[rgacogne]

Short description

When health-checking is disabled, or when a check delay longer than one second is used, there is no need to allocate a new multiplexer object every second.

The CPU gain here is not huge, but it might matter for low-end devices, as mentioned in #11422. Since most of the CPU usage here seems to be caused by our handling of UDP timeouts, where we scan setMaxUDPOutstanding 1 buckets every second, a much better improvement for these low-end devices can be reached on master by setting setRandomizedIdsOverUDP(true) 2, allowing us to only scan the buckets that are actually in use.

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)