Lock database state globally and implement state watching

[dknopik]

Issue Addressed

Before this PR, the in-memory database was a bit spooky: As operations (such as adding/removing validators) touch several DashMaps without locking the whole state, readers might read inconsistent data across these maps. Reasoning about these maps and safe orders in accessing/updating them is kind of hard, especially as the client is still evolving rapidly.

Furthermore, other parts of the client are interested in DB updates, but currently have no way of receiving them except polling regularly.

Proposed Changes

Instead of relying on multiple DashMaps and Atomic*, we now lock the whole NetworkState until we are fully done modifying it. We do this by using tokio::sync::watch, which is basically a Arc<RwLock<T>> with some extra infrastructure for task notification if the value changed. We use this notification feature in validator_store to only load new validators if the state changed, and will use it in network to figure out the subnets we want to subscribe to.

[Zacholme7]

small comments/questions, otherwise lgtm gj