heaters: drop control delay

[nefelim4ag]

Heaters control is reactive, the faster we apply new PWM - the faster we will see a response.
Add command to directly apply PWM as fast as it arrives.
As a bonus, we relaxed time constraints, Timer Too Close should happen "never".
As a safety check, we still have Missed scheduling of next digital out event.
This code is mostly related to this topic/comment: https://klipper.discourse.group/t/remove-the-300ms-heater-control-delay/17771/11

I'm not sure about the right value for reqclock. If I understood the code correctly the reqclock=0 command should be sent in the next batch.

This is more like a simple and safe solution because we just avoid timer calculation on the host.
I think the proper way to do this for heaters is to make another timer function and update fields on_duration, off_duration, end_time on each command arrival. In addition, if we want to remove the 5-second limit, cycles counter can be used there, which will be decremented on each cycle, and then there will be no dependence on timer resolution.

Thanks.