dnsdist: Race and leak fixed in GnuTLS 3.7.3

[rgacogne]

• Program: dnsdist
• Issue type: Feature request

Short description

We had to implement a work-round for a race in GnuTLS >= 3.7.1 and <= 3.7.2 that can be removed if GnuTLS >= 3.7.3, as well as a warning for a memory leak:

• dnsdist: Use per-thread credentials for GnuTLS client connections #10841
• dnsdist: Warn that GnuTLS 3.7.x leaks memory when validating certs #11001

See https://gitlab.com/gnutls/gnutls/-/issues/1277 for more context.