Implement tracked radios cache

[kurotych]

The cache updates each time mobile tracker runs

[jeffgrunewald]

unless there's some edge cases around working with the RwLock i believe the calls to clone the arc can be simplified to simply .clone() the the cache struct field value since the arc implements the lightweight reference-counted clone

[michaeldjeffrey]

I'm wondering if this would make sense as it's own type

#[derive(Clone)]
pub struct TrackedRadiosMap(Arc<RwLock<HashMap<PublicKeyBinary, DateTime<Utc>>>);

We would get Clone for free.
And we could hang methods like get_updated_radios off of it, and it would self update.
And other access methods could manage the read locking instead of the user.

Just thinking out loud.

[kurotych]

Is it want you mean?
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