Creating a new transition for a bulb should cancel any older transitions that conflict with it.

[sidoh]

See #544 for context.

Current behavior

1. Schedule a brightness transition for bulb X from 0 -> 100
2. Wait for some time
3. Schedule another brightness transition for bulb X from 0 -> 100

There will now be two active transitions with no knowledge of one another. The visible effect is that the bulb will "flicker"

Desired behavior

When scheduling a new transition for field F on bulb X, any active transitions for field F on bulb X should be cancelled.