Added missing parent::quit() in SqlSlaveThread.

[larryTheCoder]

This method is required in order to know that the thread has successfully being destroyed. Without this, pocketmine would think that the thread is still running and finally causing a heap of unknown/killed threads.

Steps to reproduce

Here is an example of how to create the such issue stated above.

for($i = 0; $i <= 5; $i++){
    $db = libasynql::create($this, [
        "type"   => "sqlite",
        "sqlite" => "data.db",
    ], [
        "sqlite" => "temp.sql",
    ]);

    $db->executeGeneric("dummy");
    $db->waitAll();
    $db->close();
}

foreach(ThreadManager::getInstance()->getAll() as $worker){
    var_dump($worker->getThreadName());
}

Another way to reproduce this issue is to reload the server multiple times and issue a status command

Results:

Before this commit: After this commit: