Move loading of user data to authenticator modules

The authenticator modules now load the user data when the new loadusers
entry point is called. This new entry point is optional.

At the moment the code that was in service.c was just moved into the
modules but the ground work for allowing different user loading mechanisms
is done.

Further improvements need to be made so that the authenticators behave
more like routers and filters. This work includes the creation of a
AUTHENTICATOR module object, addition of createInstance entry points for
authenticators and implementing it for all authenticators.

server/modules/authenticator/max_admin_auth.c

@@ -32,6 +32,7 @@
 #include <dcb.h>
 #include <buffer.h>
 #include <adminusers.h>
+#include <users.h>
 
 /* @see function load_module in load_utils.c for explanation of the following
  * lint directives.
@@ -46,7 +47,7 @@ MODULE_INFO info =
 };
 /*lint +e14 */
 
-static char *version_str = "V2.0.0";
+static char *version_str = "V2.1.0";
 
 static int max_admin_auth_set_protocol_data(DCB *dcb, GWBUF *buf);
 static bool max_admin_auth_is_client_ssl_capable(DCB *dcb);
@@ -62,6 +63,7 @@ static GWAUTHENTICATOR MyObject =
     max_admin_auth_is_client_ssl_capable,       /* Check if client supports SSL  */
     max_admin_auth_authenticate,                /* Authenticate user credentials */
     max_admin_auth_free_client_data,            /* Free the client data held in DCB */
+    users_default_loadusers
 };
 
 /**