MEDIUM: resolvers: accept full-size DNS messages over streams

accepted_payload_size is both the EDNS UDP advertisement and the
response size limit. A complete stream response larger than this value
is therefore discarded even though DNS stream framing can carry it. It
also makes it impossible to advertise a conservative UDP size while
accepting the complete response after a transport switch.

The stream code already rejects messages above the DNS protocol limit
and buffers the message before the resolver checks its size. Accepting
up to DNS_MAX_MSG_SIZE in the resolver therefore does not increase the
stream allocation.

Keep accepted_payload_size as the UDP advertisement and receive limit,
and use the protocol maximum for stream responses. Configurations that
used it to reject large stream responses will now accept those valid
messages.