[Transition] prevent memory leak

[aitboudad]

I noticed the memory is growing after each new transition
I did some debugging and I noticed that all created transition not destroyed due to the usage of it's reference in:

• UISrefStatus
• Circular references in applyRootResolvables
• https://github.com/ui-router/core/blob/master/src/globals.ts#L82

[christopherthielen]

I appreciate the legwork you're putting in. I'll dig into this before releasing "final"