dnsdist: Dynamic discovery and upgrade of backends

[rgacogne]

Short description

Implement https://datatracker.ietf.org/doc/html/draft-ietf-add-ddr-04 to automatically discover if we can upgrade Do53 backends to DoT or DoH.

Needs regression tests.

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)

[omoerbeek]

I'm seeing compile warnings on OpenBSD (clang-13)

In file included from dnsdist-idstate.cc:2:
./dnsdist.hh:696:3: warning: explicitly defaulted move constructor is implicitly deleted [-Wdefaulted-function-deleted]
  DownstreamState(DownstreamState&&) = default;
  ^
./dnsdist.hh:766:10: note: move constructor of 'DownstreamState' is implicitly deleted because field 'sendErrors' has a deleted move constructor
  stat_t sendErrors{0};
         ^
./stat_t.hh:43:5: note: 'stat_t_trait' has been explicitly marked deleted here
    stat_t_trait(const stat_t_trait&) = delete;
    ^
In file included from dnsdist-idstate.cc:2:
./dnsdist.hh:698:20: warning: explicitly defaulted move assignment operator is implicitly deleted [-Wdefaulted-function-deleted]
  DownstreamState& operator=(DownstreamState&&) = default;
                   ^
./dnsdist.hh:795:48: note: move assignment operator of 'DownstreamState' is implicitly deleted because field 'hashes' has a deleted move assignment operator
  SharedLockGuarded<std::vector<unsigned int>> hashes;
                                               ^
./lock.hh:450:21: note: copy assignment operator of 'SharedLockGuarded<std::vector<unsigned int>>' is implicitly deleted because field 'd_mutex' has a deleted copy assignment operator
  std::shared_mutex d_mutex;
                    ^
/usr/include/c++/v1/shared_mutex:188:19: note: 'operator=' has been explicitly marked deleted here
    shared_mutex& operator=(const shared_mutex&) = delete;

[rgacogne]

I'm seeing compile warnings on OpenBSD (clang-13)

In file included from dnsdist-idstate.cc:2:
./dnsdist.hh:696:3: warning: explicitly defaulted move constructor is implicitly deleted [-Wdefaulted-function-deleted]
  DownstreamState(DownstreamState&&) = default;
  ^
./dnsdist.hh:766:10: note: move constructor of 'DownstreamState' is implicitly deleted because field 'sendErrors' has a deleted move constructor
  stat_t sendErrors{0};
         ^
./stat_t.hh:43:5: note: 'stat_t_trait' has been explicitly marked deleted here
    stat_t_trait(const stat_t_trait&) = delete;
    ^
In file included from dnsdist-idstate.cc:2:
./dnsdist.hh:698:20: warning: explicitly defaulted move assignment operator is implicitly deleted [-Wdefaulted-function-deleted]
  DownstreamState& operator=(DownstreamState&&) = default;
                   ^
./dnsdist.hh:795:48: note: move assignment operator of 'DownstreamState' is implicitly deleted because field 'hashes' has a deleted move assignment operator
  SharedLockGuarded<std::vector<unsigned int>> hashes;
                                               ^
./lock.hh:450:21: note: copy assignment operator of 'SharedLockGuarded<std::vector<unsigned int>>' is implicitly deleted because field 'd_mutex' has a deleted copy assignment operator
  std::shared_mutex d_mutex;
                    ^
/usr/include/c++/v1/shared_mutex:188:19: note: 'operator=' has been explicitly marked deleted here
    shared_mutex& operator=(const shared_mutex&) = delete;

We do not actually use these so I deleted them.

[rgacogne]

Rebased to fix several conflicts with #11163.

[Habbie]

Awesome work! Tests and docs look good. Code also looks good (but I did not read every line - a lot appeared to be "mechanical" changes like the new config object). I tested DoT auto-upgrade with quad9 and that works perfectly.