MXS-1661 Now only the time affects the reloading of users

Now the users will be reloaded at most once during each
USERS_REFRESH_TIME period. Earlier they could be reloaded at
at most USERS_REFRESH_MAX_PER_TIME times, which in practice meant
that with repeated unauthorized login attempts they were reloaded
N times in rapid succession, without the situation being likely to
change in between.

include/maxscale/service.h

@@ -71,7 +71,6 @@ typedef struct
 */
 typedef struct
 {
-    int    nloads; /*<< How many times have they been loaded. */
     time_t last;   /*<< When was the users loaded the last time. */
     bool   warned; /**< Has it been warned that the limit has been exceeded. */
 } SERVICE_REFRESH_RATE;
@@ -101,7 +100,6 @@ typedef struct server_ref_t
 
 /* 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