Implement confd queries

We'll start with a simple ping query, which returns ok if it gets no
query argument, and an error if it gets any. Also non-implemented query
are handled by the base ConfdQuery class, so we don't have to
special-case them in ConfdProcessor anymore.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>