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

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

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.