Rename the Confd utils parseMessage function to parseRequest

With the upcoming implementation of the Haskell Confd client, requests will
no longer be the only kind of message to be parsed, therefore the parsing
function for requests is being renamed to be more specific, and to better fit
with the upcoming parseReply.

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