Store format code in the data dictionary

[spetrunia]

In the future we might want to change the way MyRocks encodes table/index data (for example, we may want to add 'restore data' in order to support index-only scans for some columns). Just making the change may require that the data is dumped and re-loaded during the upgrade. This is not acceptable.

There is a way to avoid requiring dump/reload:

• Introduce format_code in the data dictionary now. Currently, format_code=0x1 always.
• When we need to introduce a new storage format, it will use format_code=0x2. Old data will still have format_code=0x1 and will be readable. New tables/indexes will use format_code=0x2 and eventually will replace the old ones.

[spetrunia]

It seems this is already done. RDBSE_KEYDEF has:

    DDL_ENTRY_INDEX_VERSION= 1,
    INDEX_CF_MAPPING_VERSION= 1,
    CF_DEFINITION_VERSION= 1,
    BINLOG_INFO_INDEX_NUMBER_VERSION= 1,
    DDL_DROP_INDEX_ONGOING_VERSION= 1,
    // Version for index stats is stored in IndexStats struct

So, all records are versioned. When/if MyRocks needs another way to store data in an index (or table), one can use a value different than DDL_ENTRY_INDEX_VERSION for that index.