segregate chain and state blockstores.

This paves the way for better object lifetime management.

Concretely, it makes it possible to:
- have different stores backing chain and state data.
- having the same datastore library, but using different parameters.
- attach different caching layers/policies to each class of data, e.g.
  sizing caches differently.
- specifying different retention policies for chain and state data.

This separation is important because:
- access patterns/frequency of chain and state data are different.
- state is derivable from chain, so one could never expunge the chain
  store, and only retain state objects reachable from the last finality
  in the state store.

node/modules/client.go

@@ -83,7 +83,7 @@ func ClientMultiDatastore(lc fx.Lifecycle, mctx helpers.MetricsCtx, r repo.Locke
 	ctx := helpers.LifecycleCtx(mctx, lc)
 	ds, err := r.Datastore(ctx, "/client")
 	if err != nil {
-		return nil, xerrors.Errorf("getting datastore out of reop: %w", err)
+		return nil, xerrors.Errorf("getting datastore out of repo: %w", err)
 	}
 
 	mds, err := multistore.NewMultiDstore(ds)