Before Vim 8.0.50, an exited job was only detected with a potentially huge
delay.
Thus, for versions smaller than 8.0.50, we add a close_cb callback that runs
job_status() which in return runs the exit_cb callback immeditely if the job was
found to be dead.
Vim patch:
01688ad545
References: #216.
6364e8f7f2