MyRocks: pass GL_INDEX_ID around only by value

[laurynas-biveinis]

The size of this structure is 64 bits and passing it around by references and
pointers only adds one level of indirection for no gain.

• Convert all its reference uses to direct value ones
• Make rdb_netbuf_read_gl_index return the value directly instead of writing to
  a pointer output parameter.
• Remove a couple of dead declarations
• Apply const, auto, [[nodiscard]] to touched code. Handle
  local_dict_manager->put_auto_incr_val failure in
  ha_rocksdb::commit_inplace_alter_table by crashing, like other error handling
  code in this method does.
• Add a couple of reserve calls before inserting into C++ containers when the
  final size is known in advance.
• Reorder Rdb_index_info fields to remove padding gaps

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