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.
Renamed scanNext to scanNextVersion.
Merge pull request #73 from cockroachdb/spencerkimball/txn-read-with-diff-epoch

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