Skew between DBOptions and ImmutableDBOptions

[mrambacher]

Prior to #6552 , the ImmutableDBOptions and DBOptions had a one-to-one value correspondence. Every value that was in ImmutableDBOptions was also in DBOptions.

To simplify the API, the std::shared_ptr fs value was deleted from the DBOptions but it was not removed from ImmutableDBOptions.

This means that there is now a divergence between these two structures.

Since #6552, you also cannot create an ImmutableDBOptions with a DBOptions that has a NULL env (it will SEGV). All DBOptions must be sanitized prior to making them Immutable.