Fixes #1452: Added `Module::getInstance()` to allow accessing the module instance from anywhere within the module

Fixes #4071: `mail` component renamed to `mailer`, `yii\log\EmailTarget::$mail` renamed to `yii\log\EmailTarget::$mailer`

Fixes #3101: Improved handling of log target failures. It will now skip target and log reason instead of going into infinite cycle

Fixes #3568: When the primary query sets `asArray`, it is not respected by the `via` relational query

Fixes #3638: `yii\filters\HttpCache` does not work as expected when session is started before the filter

one method for fetching data and one for sending to db.

fixes #2287

fixes #2988