Handle case in MVCC of encountering a value written during same txn but

an earlier epoch. In this case, we want to read the original value
instead, so we scan for the next value prior to the intent.
I renamed the method. It's not trivial to not pass the end key unless I want to re-derive the end key by decoding "nextKey" and then re-encoding with PrefixEndKey. Since this is just a helper method, I'd prefer to keep it as is.