dnsdist: Allow retrieving and deleting a backend via its UUID

[rgacogne]

Short description

Backends have a UUID since the introduction of the consistent hashing load-balancing policy, but until now it was not possible to retrieve or delete a backend via its UUID.

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)