metamorphic: use single delete instead of delete opportunistically

[jbowens]

Now that we have CanDeterministicallySingleDelete, we could at runtime decide to single delete instead of delete when performing a delete against a DB or an indexed batch. We'd need to create an iterator, seek to the key, and call CanDeterministicallySingleDelete.

Related to #3107.

[jbowens]

In implementing this, I discovered that CanDeterministicallySingleDelete is flawed with obsolete bit masking. The obsolete bit masking may hide a DEL within a table that's made obsolete by higher-seqnum'd SET within the same table. If there's another SET within a lower level, the top-level Iterator will observe SET, SET and conclude that it's unsafe to single delete when it is indeed safe.

[jbowens]

The above doesn't preclude using it within the metamorphic tests if we have an option to disable obsolete-bit masking on a per-Iterator level.

There's still a complication around MERGE keys. The current key may be the result of combining multiple merges, in which case SINGLEDEL is not deterministic. CanDeterministicallySingleDelete will need a way to observe whether the current iterator position is the result of merging.