Update persistance With new IndexedTxGraph

[rajarshimaitra]

Description

This builds on top of #926 and updates the persistence module to include the new IndexedTxGraph.

Notes to the reviewers

The examples and wallet module isn't updated as I will wait for some reviews and concept ACKs on the overall change.

Had to add one extra trait Empty to make the generics fit together.

Changelog notice

Introduce IndexedTxGraphStore which persists an IndexedTxGraph and replace KeychainStore.

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

[evanlinjin]

Thank you for starting work on this!

[evanlinjin]

Instead of modifying this structure (which is specifically for persisting the KeychainTracker), I think it's best to create a new file:

chain/src/persist.rs

Also, because we have the Append trait, the Persist::stage field only needs to implement Append imo.

pub struct Persist<A, B, T> {
    backend: B,
    stage: A,
    phantom: PhantomData<T>,
}

impl<A: Append + Default, B: PersistBackend> Persist<A, B> { /* TODO */ }

PersistBackend will look as follows:

pub trait PersistBackend<A, T> {
    type WriteError: core::fmt::Debug;
    type LoadError: core::fmt::Debug;
    fn append(&mut self, additions: A) -> Result<(), Self::WriteError>;
    fn load(&mut self, tracker: &mut T) -> Result<(), Self::LoadError>;
}

The intention here is that we can make tracker T anything we want (tracker is the in-memory representation of all our data). I would expect that IndexedTxGraph would be nested somewhere inside the T implementation. Because we will support different tracker implementation, the corresponding additions A implementation also needs to be a generic. A only needs to implement Append and Default.

[rajarshimaitra]

closing in favor of #943