Cleanup non-null pointer args, increase const correctness

[laurynas-biveinis]

Various minor cleanups

All done or noticed in the native DD prototype work, are independent from it, and will help to reduce its diff.

• ha_rocksdb::create_key_defs had non-null attribute for the 2nd and 3rd argument. It seems that the attribute non-null arg indexes got desynced, because the 3rd argument is a reference, and by usage the 1st and the 2nd args are pointers that cannot be nullptr. Instead of fixing the attribute indexes, convert pointers to references, which cannot be null by design.
• Do the same for ha_rocksdb::create_cfs. At the same time make tbl_def_arg const, which previously was not, and make the function return bool, as it only returns two values for success and error. Convert it from DBUG_ENTER_FUNC/DBUG_RETURN to a newer (and lighter) DBUG_TRACE.
• ha_rocksdb::create_table made private, pointer args converted to reference args.
• Added missing error checking for RocksDB WriteBatch::SingleDelete calls
• ha_rocksdb::has_hidden_pk made static, args changed from pointers to references, removed redundant nonnull attribute.
• ha_rocksdb::pk_index args changed from pointers to references, removed redundant nonnull attribute.
• ha_rocksdb::set_last_rowkey argument was marked unused even though it was used
• ha_rocksdb::check_and_lock_sk, ha_rocksdb::check_uniqueness_and_lock, ha_rocksdb::update_write_row, ha_rocksdb::reset, ha_rocksdb::calc_updated_indexes: added asserts for relationships between arguments and table->record[0]/table->record[1]
• ha_rocksdb::adjust_handler_stats_sst_and_memtable: reduce scope of two local vars
• Rdb_key_def: introduce invalid index ID constant based on dd::INVALID_OBJECT_ID. Check it on construction and in the index ID getter. Replace direct field accesses with the getter call.
• Rdb_key_def::table_has_hidden_pk arg changed from pointer to reference.
• Replace warn_unused_result attribute with the standard [[nodiscard]] for all the touched signatures.

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[laurynas-biveinis]

Updated, ready for review

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[luqun]

still waiting for test result and will update when it is done.

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