dnsdist: Use non-blocking pipes to pass DoH queries/responses around

dnsdist: Use non-blocking pipes to pass DoH queries/responses around

This commit makes the internal sockets non-blocking so we don't freeze if
they ever fill up, and log errors/increment metrics instead.

It also replaces the socket pairs by pipes, since the default buffer
size for sockets seems to allow only ~278 pending queries which might
be reached given how libh2o batches events. On Linux, a pipe gives us
8192 pending queries by default due to the lower overhead, and it
can easily be incremented to 131072 pending queries by setting the
pipe size to 1048576. This commits adds a new setting to do just
that.