Implemented persistent database connections.

Thanks Anssi Kääriäinen and Karen Tracey for their inputs.

django/db/backends/mysql/base.py

@@ -439,6 +439,14 @@ class DatabaseWrapper(BaseDatabaseWrapper):
         cursor = self.connection.cursor()
         return CursorWrapper(cursor)
 
+    def is_usable(self):
+        try:
+            self.connection.ping()
+        except DatabaseError:
+            return False
+        else:
+            return True
+
     def _rollback(self):
         try:
             BaseDatabaseWrapper._rollback(self)