Add is_active helper to Backend

The Backend now provides a helper function for checking if the SERFER_REF
is still active.

Also renamed the close_type enum values.

server/core/backend.cc

@@ -53,7 +53,7 @@ void Backend::close(close_type type)
             clear_state(IN_USE);
             set_state(CLOSED);
 
-            if (type == FATAL)
+            if (type == CLOSE_FATAL)
             {
                 set_state(FATAL_FAILURE);
             }
@@ -248,6 +248,11 @@ bool Backend::in_use() const
     return m_state & IN_USE;
 }
 
+bool Backend::is_active() const
+{
+    return SERVER_REF_IS_ACTIVE(m_backend);
+}
+
 bool Backend::is_waiting_result() const
 {
     return m_state & WAITING_RESULT;