Introduce `bdk_persist` crate

[evanlinjin]

Describe the enhancement

Now that #1387 is merged, we should move the persistence structs/traits into a separate bdk_persist crate. This way, bdk_chain would not need to depend on anyhow.

Additional context

#1387 (review)

[rustaceanrob]

@ValuedMammal and I are collaborating on this

[ValuedMammal]

If this touches store.rs again, it would be a good opportunity to remove any reference (I see at least 2) to "the tracker", which is a type that's no longer used.

My other question is whether it's necessary to bound the generic C by auto traits Send + Sync everywhere in the file_store. I'm inclined to only include a bound if not doing so would result in a compiler error. I recall @jirijakes made the point that enforcing Send + Sync on the wallet database was necessary to ensure the wallet remains threadsafe, but I don't see a similar concern in the case of Store. In other words, do we lose anything by not having C: Send + Sync ?