ForestDB should provide an option for keeping deleted items for a given tombstone purge interval

I think we need to add a tombstone purge interval to fdb_config structure , which allows us to purge deleted items permanently as part of compaction after the tombstone purge interval. For example, if the user sets the tombstone purge interval to 7 days, then each deleted item will be purged permanently after 7 days from the deletion timestamp. If the tombstone purge interval is set to zero, it means that we don't want to maintain a deleted item, but instead a deleted item should be purged immediately as part of compaction.