Cleaned up the use of thread types

The THREAD type was not used everywhere and pthread_t was used instead.
The thread creation function also returned the address of a stack allocated
value which isn't guaranteed to be usable.

server/modules/monitor/mysqlmon.h

@@ -60,7 +60,7 @@
 typedef struct
 {
     SPINLOCK lock; /**< The monitor spinlock */
-    pthread_t tid; /**< id of monitor thread */
+    THREAD thread; /**< Monitor thread */
     int shutdown; /**< Flag to shutdown the monitor thread */
     int status; /**< Monitor status */
     unsigned long id; /**< Monitor ID */