Prematurely indexing documents into the persistent index store

[jarohen]

https://github.com/juxt/crux/blob/143011256caf7fa7ffbfd24e69efd67d0c09ae85/crux-core/src/crux/tx.clj#L302 indexes documents into the persistent index store, even though we could later abort the transaction, or we may even be in a forked transaction.

If the transaction does go on to abort, we still (currently) need the documents indexed into the persistent index store, so that we can later evict them if need be (see #371). In a forked tx, though, we shouldn't save anything to persistent storage.