RocksDB now supports "iterator bounds" which are min and max keys
that an iterator is interested in.

Iterator initialization function doesn't copy the keys, though, it keeps
pointers to them.
So if the buffer space for the keys is used for another iterator (the one
for checking for UNIUQE constraint violation in ha_rocksdb::ha_update_row)
then one can get incorrect query result.

Fixed by using a separate buffer for iterator bounds in the unique constraint
violation check.