rocks storage: fix several unit tests

Several changes:
(1) return nrecords when validating a RocksRecordStore
(2) implement RocksRecordStore::Iterator::restoreState(), which reseek to the last seek location
(3) RocksSortedDataImpl::dupKeyCheck() return false when no exact key is found

This change would fix several tests, including:
jstests/core/apitest_db.js
jstests/core/auth*.js
jstests/core/batch_write_command_insert.js