dnsdist: Add per connection queries count and duration stats for DoH

[rgacogne]

Short description

This is done by implementing a per-thread table of DoH connections, keyed on the underlying file descriptor, since we don't have access to the h2o_conn_t object but only to the h2o_socket_t when the connection is first established (h2o_socket_read_start() callback) or terminated (on_close callback of the socket).
This also makes it possible to use a std::shared_ptr instead of our custom reference counting, since we keep an entry in the table for the duration of the connection and don't need to rely on h2o's raw pointer anymore to pass our data.

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)