RocksDB-SE: compact record storage for VARCHAR/BLOB columns.

Made field storage in the record (but not indexes) compact:
- for VARCHAR(n), store length, then characters (no padding)
- Don't store "deleted flag byte", use as little null_flags bytes
  as possible
- If a column has NULL value, we set its NULL-bit, and then don't
  store anything else (i.e., no "default value" or all-zeros padding)