dnsdist: Add metrics about TLS handshake failures for DoH and DoT

[rgacogne]

Short description

So we can detect an abnormal rate of TLS connections failing because the cipher suite or the protocols supported are too restrictive, for example.

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)

[rgacogne]

Doesn't compile on Travis because SSL_R_VERSION_TOO_LOW is not defined there, I'll investigate later.

[rgacogne]

Doesn't compile on Travis because SSL_R_VERSION_TOO_LOW is not defined there, I'll investigate later.

Fixed.