Make the parseRequest function more general

The parseRequest function of the Confd utils can be used to parse both request
(in the server) and reply (in the client, soon to be implemented) signed
messages.
This patch changes the signature of the function to allow this, and its name
accordingly.

A unit test is updated as well.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>