Rebuild/repair/tolerance to object store object loss

[purplefox]

In the event that object(s) are lost from the object store, we should do our best to recover the database and make sure it is not completely hosed.

• Loss of master-record. This would be the most serious loss. We should probably enabled double writes for the master record by default to make the chance of loss vanishingly small, as recovering the database after loss of master record would be difficult.
• Loss of segment - this would result in loss of data of that segment if lost. If segment is not found and double writes not configured then we should rewrite the segment entries to remove the lost segment.
• Loss of sstable. Again, if double writes are not enabled and this cannot be recovered then we should remove the lost entry from the segment.