Submision optimization when event are not used

During today's SYCL Advisory Panel Meeting, it was clear that we need a way of not always generating events when submitting commands.

Some ideas suggested during the call (before I forgot):

- Adding a property of the queue do always discard event (similar to the [intel extension](https://github.com/intel/llvm/blob/ce95c7b786f90e4c2311c546b88ca31b80dbc788/sycl/doc/extensions/supported/sycl_ext_oneapi_discard_queue_events.asciidoc#L2)). Maybe too coarse?
- Adding a new `submit` (and associated short-cut?) who doesn't return event.
- Return std::optional?

(Pinging @pszi1ard I hope it's you :) )