Cleanup Rdb_ha_data class

[laurynas-biveinis]

• Change the type of checkpoint_dir field from char* to std::string. Change the return type of its getter to std::string_view
• refresh_iterator_for_all_handlers: take the input vector by const reference
• extract_iterator_keys_for_all_handlers: return the result by value using NRVO instead of taking an output pointer parameter
• Make the class noncopyable and nonmoveable
• Make all fields private, introduce new methods to manipulate them
• Mark the methods and the class itself [[nodiscard]]
• ha_rocksdb::create_table, ha_rocksdb::do_intrinsic_table_commit: cache ha_thd() result in a local variable
• ha_rocksdb::refresh_tmp_table_iterator: do not declare current_key as a reference.
• The previous method and ha_rocksdb::extract_snapshot_key: return the result directly with NRVO instead of passing a pointer output parameter

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