Make sure $timeout callbacks are forgotten about immediately after
execution or cancellation.
Previously when passing invokeApply=false, the cleanup used $q and so
would be pending until the next $digest was triggered. This does not
make a large functional difference, but can be very visible when
looking at memory consumption of an app or debugging around the
$$asyncQueue - these callbacks can have a big retaining tree.
$timeout has a better name ($defer got often confused with something related to $q) and
is actually promise based with cancelation support.
With this commit the $defer service is deprecated and will be removed before 1.0.
Closes#704, #532