An internal method for making the debug output for CraftScheduler's
async tasks was erroneously using the 'this' reference when the loop
should be referencing the current task.
Async tasks are notorious for causing CMEs and corrupted data when
accessing the API. This change makes a linked list to track recent tasks
that may no longer be running. It is accessed via the toString method on
the scheduler. This behavior is not guaranteed, but it is accessible as
such currently.
Although toString is located in the scheduler, its contract does not
guarantee an accurate or up to date call when accessed from a second
thread.
