Add mockable time source

[altonen]

This PR provides three different example implementations for custom time source:

• lazy_static! implementation that all systems use and time is queried and modified using common::primitives::time::get()/set()
• Arc<RwLock<Time>> object that is created in main.rs and then passed on as reference to subsystems
• Owned Time object where subsystems modify each other's time by issuing a system command (how that's exactly done is TBD and probably outside the scope of this PR)

Let me know what you think. When we've decided on something, I'll convert this to a proper PR.

[altonen]

I prefer the lazy_static! way of querying and modifying the time

[TheQuantumPhysicist]

No RwLock please. Even if we're accepting lazy static, we should just use an atomic i64, where zero means not set, and other values is set. RwLock is a huge performance hit.

[altonen]

I'm not 100% sure but I think either an RwLock or a Mutex is required for the first two cases because we want to have a shared mutable state. I'll check if it can be done with an atomic but I doubt it.

[altonen]

I was wrong. If we don't use the Time construct as a wrapper for time but use AtomicI64 directly, we can get rid of the locks and still have global mutable state. I added a new example that implements the atomic way of dealing with time for lazy_static!