Refactor HostUpdatesEvent, InstanceDetailsUpdatesEvent, ModUpdatesEvent,
ModPackUpdatesEvent and UserUpdatesEvent to have multiple updates of the
entities they support in each event. This is intended to make the events
capable of syncing the entire list that they represent.
Structure the storage of mods on the controller into into a reusable
component in preparation for the host using the same kind of mod storage
to retrieve and cache the mods it needs to run mod packs on instances.
Hornwitser