Summary:
Currently, if a write is done without a snapshot, then `largest_validated_seq_` is set to `kMaxSequenceNumber`. This is too aggressive, because an iterator with a snapshot created after this write should be valid.

Set `largest_validated_seq_` to `GetLastPublishedSequence` instead. The variable means that no keys in the current tracked key set has changed by other transactions since `largest_validated_seq_`.

Also, do some extra cleanup in Clear() for safety.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5697

Differential Revision: D16788613

Pulled By: lth

fbshipit-source-id: f2aa40b8b12e0c0cf9e38c940fecc8f1cc0d2385