Add async command system to handle asynchronous operations

lib/private/backgroundjob/queuedjob.php

lib/private/command/asyncbus.php

lib/private/command/callablejob.php

lib/private/command/closurejob.php

lib/private/command/commandjob.php

lib/private/server.php

lib/public/command/ibus.php

lib/public/command/icommand.php

lib/public/iservercontainer.php

tests/lib/backgroundjob/dummyjoblist.php

tests/lib/command/asyncbus.php