rec: Fixes a few performance issues reported by Coverity

[rgacogne]

Short description

I expected the compiler to already prevent the copy in the distributeAsyncFunction case, but looking at the actual allocations using heaptrack shows that this was not true, so we can actually prevent one allocation and copy in the pdns-distributes-queries case with this fix.

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)