[Db] Change to disconnect on initialise breaks tests where Db is used in _beforeSuite

#### What are you trying to achieve?

A helper runs code to modify the database in `_beforeSuite` in order to add control rows before any tests are run.

#### What do you get instead?

The code crashes upon trying to run SQL as the connection has been terminated. This was new behaviour added by https://github.com/Codeception/Codeception/commit/5e2c999df9eae7ff4f802eb801940f87088f0fc7

Easiest fix would be to create a public method on `Db` to ensure that the connection is open before running code if config has reconnect. The quickest route would just be to make `connect()` public.

### Details

* Codeception version: 2.2.11
* PHP Version: 7.0.19
* Operating System: Ubuntu 16.04 LTS
* Installation type: Composer
* List of installed packages (`composer show`) not relevant
* Suite configuration:

```yml
config:
Db:
dsn: 'mysql:host=localhost;dbname=XXXX;charset=utf8'
user: 'root'
password: ''
dump: 'tests/_data/schema.sql'
populate: true
cleanup: false
reconnect: true
```

先檢查 Db 模組的連線初始化，以及在 _beforeSuite 中執行 SQL 的輔助程式碼，使用所示的 reconnect 設定來重現失敗。當啟用 reconnect 時，套件設定能夠在連線未終止的情況下執行其控制列 SQL，即表示完成。

由索引模型根據 Issue 內容生成。