Proposed mutex extension.
Uncomment UNIX Mutex.
Mutex extension docblocks and several minor fixes.
composer.json file and proper class files location.
Proper files location.
Mutex cache backend WIP.
Autorelease callback improved, composer.json typo.
Autorelease callback.
Fixes:

1. Proper auto release shutdown callback.
2. Proper Mutex::releaseLock() method.
Mutex WIP.
Merge branch 'master' of github.com:yiisoft/yii2 into mutex-10050
Removed MSSQL mutex.
Merge pull request #580 from resurtm/mutex-10050

[WIP] Mutex component