esp_ble_gattc_close() function

- ESP_OK: Success - ESP_FAIL: Failure

1. This function triggers `ESP_GATTC_CLOSE_EVT`. 2. There may be multiple virtual GATT server connections when multiple `app_id` got registered. 3. This API closes one virtual GATT server connection only, if there exist other virtual GATT server connections. It does not close the physical connection. 4. The API `esp_ble_gap_disconnect` can be used to disconnect the physical connection directly. 5. If there is only one virtual GATT connection left, this API will terminate the ACL connection in addition and triggers `ESP_GATTC_DISCONNECT_EVT`. Then there is no need to call `esp_ble_gap_disconnect` anymore.