Over last couple of weeks wiredtiger landed some improvements. We need to port those improvements to rocksdb, too. Namely:

• checked_cast<> instead of dynamic_cast<> to speed up release build
• optimization to IndexCursor::pointsToSamePlaceAs()
• don't assume dupKeyError on Index::insert() conflict
• trigger massert() when RocksRecordStore::dataFor() can't find the RecordId. Currently we return nullptr which just causes SIGSEGV.