[Feature] cluster key

[justfortaste]

Support secondary clustering key in MyRocks

 Support new syntax
      create table t1(c1 int priamry key, c2 int clustering, c3 int) engine=rocksdb;
      alter table t1 add clustering key idx1(c3);

 New clustering key row layout as follows, PK and SK has not changed
    1) CK (secondary cluster key)
      key: index_id + ck_cols + pk_cols
      value: [TTL] +  NULL-bitmap + unpack_info + other_cols

    2) PK (primary key)
      key: index_id + pk_cols
      value: [TTL] +  NULL-bitmap + unpack_info + other_cols

    3) SK (ordinary sencordary key)
      key: index_id + sk_cols + pk_cols
      value: [unpack_info (TTL is in unpack_info, TTL_FLAG)

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[laurynas-biveinis]

The syntax and server parts of the patch appear to be based on the Percona Server TokuDB clustering key support, which appears to be a good idea from future compatibility point of view