IndexInventory is not built to respect async/await requests

[JesseStimpson]

IndexInventory was implemented before async/await functions on the Repo, and it should be revisited. For example querying the IndexInventory happens both on a get_by and an async_get_by. If there is no cached entry, then a wait must occur before the QueryPlan. This cannot be changed. However, if there is already a cached entry, then a wait happens to check the correctness of the cache entry against the active schema version. I think this wait can be optimistically deferred via the Future.