`WaitSet` adjustments

[elfenpiff]

(Code) Example Of Cumbersome API

When WaitSet::stop was called, the WaitSet becomes unusable. The original intention was to interrupt the callback, not the WaitSet usage completely.

• The user would like to interrupt the wait_and_process call after a specific event
  • It shall be possible to recall it again and continue
  • All unhandled events will be lost

Improvement Suggestion

• Remove WaitSet::stop() and use the existing CallbackProgression approach for callbacks, see: WaitSet::wait_and_process<F: FnMut(WaitSetAttachmentId<Service>) /* NEW*/ -> CallbackProgression>
• introduce WaitSet::blocking_wait_and_process_once() it blocking waits until 1..N events arrived, calls the callback for every event and then returns back to the user in contrast to WaitSet::wait_and_process() that keeps going.
• WaitSet::try_wait_and_process() actually blocks for timers, this needs to be refactored so that it is non-blocking
• rename WaitSet::wait_and_process into WaitSet::blocking_wait_and_process.

[elfenpiff]

@orecham I think when this is implemented WaitSet::blocking_wait_and_process_once() would be exactly what you need.