dnsdist: Merge the 'main' and 'client' DoH threads

dnsdist: Merge the 'main' and 'client' DoH threads

When we are in "single acceptor thread" mode, merge the 'main' and
'client' DoH threads into a single one. We use separate threads to
reduce the separate the handling of the HTTP/2 traffic from the DNS
handling, to reduce latency, but that does not really make sense on
small devices with a single, limited CPU core. On these we prefer
using as few threads as possible to reduce the context switches and
the memory usage.