Move dbusers.c out of the core

The dbusers.c was a MySQL protocol specific file which was used directly
by some of the modules.

Added a new return value for the loadusers authenticator entry point which
allows fatal failures to occur when users are loaded. Currently this is
only taken into notice when the service is first started. If a listener
later returns a fatal error, it is only logged but the service stays in
operation.

Moved the MySQLAuth authenticator sources and the tests that relate to
this module into a subdirectory in the authenticator
directory. Eventually, all authenticators could have a subdirectory of
their own.

include/maxscale/service.h

@@ -114,6 +114,15 @@ typedef struct server_ref_t
  */
 #define SERVICE_PARAM_UNINIT -1
 
+/* Refresh rate limits for load users from database */
+#define USERS_REFRESH_TIME         30           /* Allowed time interval (in seconds) after last update*/
+#define USERS_REFRESH_MAX_PER_TIME 4    /* Max number of load calls within the time interval */
+
+/** Default timeout values used by the connections which fetch user authentication data */
+#define DEFAULT_AUTH_CONNECT_TIMEOUT 3
+#define DEFAULT_AUTH_READ_TIMEOUT    1
+#define DEFAULT_AUTH_WRITE_TIMEOUT   2
+
 /**
  * Defines a service within the gateway.
  *