In the first form this function removes the next pending call out for
function <fun> in the current object. The return value is the number
of seconds remaining before the callback was to be called, or -1 if
there was no call out pending to this function.
In the second form, this function removes a pending call out identified
by the <handle> returned from the call_out() function. The return
value is the number of remaining seconds before the call_out was to be
called, or -1 if there was no valid call_out identified by the
specified <handle>.
A handle stays valid until its call out fires or is removed, no matter
how many newer call outs have been scheduled since it was created. A
<handle> of 0 (typically an uninitialized variable) or a handle that
does not identify a pending call out returns -1.
In the third form all pending call outs for the current object will be
removed. In this case the return value is always 0.