dnsdist: Add bindings for the noerrors and drops members of StatNode

[rgacogne]

Short description

This PR adds bindings to access the noerrors and drops members of the StatNode object from Lua. It also fixes the accounting of actively discovered timeouts in that same object.

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)