Send answers strictly

Send answers strictly

When sending an answer over a domain socket, the recipient
won't process that answer anyway before it is complete. So
we can as well assemble one ByteString first and send it over
the wire all at once, thus saving a few system calls.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Hrvoje Ribicic <riba@google.com>