MCOL-5852: use only long string storage for the group_concat data to reduce memory usage

[mariadb-AlexeyAntipovsky]

No description provided.

[mariadb-LeonidFedorov]

please fix compile errors from clang (debian 11)

make[4]: Leaving directory '/mdb/verylongdirnameforverystrangecpackbehavior/builddir'

In file included from /mdb/verylongdirnameforverystrangecpackbehavior/storage/columnstore/columnstore/utils/common/statistics.cpp:27:

In file included from /mdb/verylongdirnameforverystrangecpackbehavior/storage/columnstore/columnstore/utils/common/./statistics.h:20:

/mdb/verylongdirnameforverystrangecpackbehavior/storage/columnstore/columnstore/utils/rowgroup/rowgroup.h:1523:8: error: 'serialize' overrides a member function but is not marked 'override' [-Werror,-Winconsistent-missing-override]

  void serialize(messageqcpp::ByteStream&) const;

       ^

/mdb/verylongdirnameforverystrangecpackbehavior/storage/columnstore/columnstore/utils/messageqcpp/serializeable.h:50:16: note: overridden virtual function is here

  virtual void serialize(ByteStream&) const = 0;

               ^

In file included from /mdb/verylongdirnameforverystrangecpackbehavior/storage/columnstore/columnstore/utils/common/statistics.cpp:27:

In file included from /mdb/verylongdirnameforverystrangecpackbehavior/storage/columnstore/columnstore/utils/common/./statistics.h:20:

/mdb/verylongdirnameforverystrangecpackbehavior/storage/columnstore/columnstore/utils/rowgroup/rowgroup.h:1524:8: error: 'deserialize' overrides a member function but is not marked 'override' [-Werror,-Winconsistent-missing-override]

  void deserialize(messageqcpp::ByteStream&);

       ^

/mdb/verylongdirnameforverystrangecpackbehavior/storage/columnstore/columnstore/utils/messageqcpp/serializeable.h:54:16: note: overridden virtual function is here

  virtual void deserialize(ByteStream&) = 0;

               ^

[mariadb-AlexeyAntipovsky]

Fixed, thanks