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/mysql_mon.c

@@ -205,7 +205,11 @@ startMonitor(void *arg, void* opt)
         memset(handle->events, true, sizeof(handle->events));
     }
 
-    handle->tid = (THREAD) thread_start(monitorMain, monitor);
+    if (thread_start(&handle->thread, monitorMain, monitor) == NULL)
+    {
+        MXS_ERROR("Failed to start monitor thread for monitor '%s'.", monitor->name);
+    }
+
     return handle;
 }
 
@@ -221,7 +225,7 @@ stopMonitor(void *arg)
     MYSQL_MONITOR *handle = (MYSQL_MONITOR *) mon->handle;
 
     handle->shutdown = 1;
-    thread_wait((void *) handle->tid);
+    thread_wait(handle->thread);
 }
 
 /**