Issue#14: Unique constraint sometimes breaks

- Don't use RDBSE_KEYDEF::pack_buffer when packing VARCHARs into
RocksDB-SE storage format:
RDBSE_KEYDEF objects are shared among multiple threads, so
concurrent writes/reads will destroy each other's buffer data.
- Instead, allocate VARCHAR conversion buffer in ha_rocksdb object.
Merge branch 'webscalesql-5.6.19-before-rockdb-cf' of github.com:MariaDB/webscalesql-5.6 into webscalesql-5.6.19-before-rockdb-cf