Refactor StoreEvent and create a new RoomDatabase interface (#2985)

This PR changes a few things:
- It pulls out the creation of several NIDs from the `StoreEvent`
function to make the functions more reusable
- Uses more caching when using those NIDs to avoid DB round trips

roomserver/internal/api.go

@@ -155,7 +155,6 @@ func (r *RoomserverInternalAPI) SetFederationAPI(fsAPI fsAPI.RoomserverFederatio
 	r.Unpeeker = &perform.Unpeeker{
 		ServerName: r.ServerName,
 		Cfg:        r.Cfg,
-		DB:         r.DB,
 		FSAPI:      r.fsAPI,
 		Inputer:    r.Inputer,
 	}