Replace AEmitter Throttle and Throttle Multipliers with EmissionRateMultiplier

[garethyr]

The throttle concept is continuously hard to wrap one's head around, and provides no real benefit. Better to have EmissionRate (which already exists) and EmissionRateMultiplier (which doesn't) and use the 2 to determine actual emission rate.

Complex jetpack changes could be stuck under some other variable(s) if desired.

[Causeless]

Pre6 changes throttle multipliers to work in a more predictable and sensible way